This is a tricky one! Here's a breakdown of potential causes and troubleshooting steps, based on the information you've provided

Possible Causes (Beyond a Simple Faulty Freezer or GFCI):
Neutral-Ground Fault

This is the most likely culprit, especially since it takes a while to trip. It could be a subtle wiring issue where a tiny amount of neutral current is finding its way to ground. This is very difficult to detect with standard tools.

Voltage Fluctuations/Surges

The appliance starts, works fine, and then a change in line voltage (sag or surge) could be creating a momentary ground fault condition that trips the GFCI. This could be intermittent.

Harmonics or "Dirty Power"

Other equipment on the same circuit or even nearby circuits could be introducing harmonics (distortions in the AC waveform) that the GFCI is misinterpreting as a ground fault.

Parallel Neutral Path

If there's another path for the neutral current to return to the panel (other than the intended neutral wire), it can cause a current imbalance that trips the GFCI. This is often linked to shared neutrals or connections to ground in other parts of the electrical system.

Grounding Issues Elsewhere

A loose or corroded ground connection somewhere else in the building's electrical system can create instability and contribute to GFCI tripping.

Faulty Appliance, but Intermittent

It's possible the freezer does have a fault, but it's very slight and only manifests after the compressor runs for a while, causing the motor windings to heat up and start leaking voltage.

Load

Even though the freezer worked well when tested using the extension cord on the other GFCI, the extension cord is extra cable that would add resistance to the circuit. When plugged into the problem GFCI maybe the extra cable resistance is just enough to keep the amperage low enough to not cause a GFCI issue in the given time.

Troubleshooting Steps (in order of likelihood and ease of implementation)


Rule Out the Appliance (Again, More Thoroughly)



Megger Test

A megohmmeter (megger) applies a high DC voltage to the appliance's wiring and measures the insulation resistance. This is much more sensitive than a simple continuity test. A reading below a certain threshold (typically 1 megohm or higher) indicates a potential insulation breakdown and a ground fault. Test both the line and neutral to ground.

Clamp Meter for Leakage Current

Use a clamp meter capable of measuring milliamp leakage current. Clamp it around both the hot and neutral wires feeding the freezer (at the same time). If there's a ground fault, even a very small one, the meter will show a reading (ideally, it should be 0, but anything above 5mA is cause for concern with a GFCI). Do this while the freezer is running and after it's been running for a while.

Temporary Dedicated Circuit (Best Test)

If possible, run a temporary dedicated circuit from a different panel to the freezer's location, and plug it into a new GFCI receptacle. This completely isolates the freezer from the existing circuit and can definitively rule out issues with the building's wiring. Make the temporary wiring as short as possible to reduce voltage drop.2.

Investigate Neutral-Ground Voltage and Bonding



Neutral-Ground Voltage Test

Use a multimeter to measure the voltage between the neutral wire and the ground wire at the problem receptacle with the freezer running. A normal reading should be very close to 0 volts (ideally, less than 1 volt). Any significant voltage (over 2-3 volts) indicates a neutral-ground fault somewhere.

Check Bonding at the Panel

Ensure the neutral bus and ground bus in the electrical panel are properly bonded (connected together). A loose or missing bonding connection is a common cause of GFCI problems. Check that the main bonding jumper is intact and properly connected.3.

Examine the Wiring (Again, Carefully)



Check All Connections

Go back and re-examine every connection in the circuit, from the breaker in the panel to the receptacle. Look for loose wires, corroded connections, or any signs of damage. Pay special attention to connections in junction boxes that might be hidden.

Verify Proper Wiring

Ensure that the wiring is correctly connected to the receptacle. The hot, neutral, and ground wires should be connected to the proper terminals.

Check Wire Insulation

Carefully inspect the insulation of the wires in the circuit. Look for any signs of nicks, cuts, or damage that could be causing a ground fault.4.

Address Potential "Dirty Power" Issues



Identify Potential Sources

Are there any other appliances or equipment on the same circuit or nearby circuits that could be generating harmonics? Examples include variable frequency drives (VFDs), electronic ballasts for fluorescent lights, and some types of computer power supplies.

Isolate Suspect Equipment

Try turning off or disconnecting suspect equipment to see if it resolves the GFCI tripping issue.

Power Conditioning

In some cases, a power conditioner or surge protector with harmonic filtering capabilities may help to reduce the effects of "dirty power."5.

Consider a Different Type of GFCI



Commercial-Grade GFCI

Some commercial-grade GFCIs are designed to be more tolerant of nuisance tripping caused by slight leakage currents or voltage fluctuations.6.

Troubleshooting questionsWas the old freezer on a GFCI and was there issues before the install of the new freezer?Did you use the exact same gauge of wiring when replacing the short wire run?

Was the new freezer in transport for any time? If it sat on its side for a period of time the freon may have migrated into the lines and cause a temp load surge.

Important Considerations



Safety First

Always de-energize the circuit before working on any electrical wiring.

Call a Qualified Electrician

If you're not comfortable working with electrical wiring, or if you've exhausted these troubleshooting steps and still can't find the problem, it's best to call a qualified electrician.

Document Everything

Keep detailed notes of your troubleshooting steps, including the results of any tests you perform. This will help you track your progress and identify potential patterns.

Codes: Ensure you're following all applicable electrical codes and regulations.By systematically working through these troubleshooting steps, you should be able to identify the cause of the GFCI tripping issue and resolve it. Let me know if you have more information or results as you go through these steps, and I'll do my best to help further. Flag for review