There is a possibility that the unit itself has some leakage current. By standards, I believe the newer appliances of today are limited in the leakage current to 1/5 the value of what would trip a standard Class 'A' GFCI device. If these appliances were designed and built before the newer standard, it is possible that it is contributing towards the GFCI tripping at a time after the equipment has been operating...maybe the heat from usuage adds to the issue.
Equipment techs do not usually test for this type of current leakage within the equipment...the portable testing equipment for this is not standard equipment even though I also believe the price has come down considerably for them.
See if you can rent a leakage current tester (mili-amps) and test them or have the equipment techs test again, including the mili-amp.
Let us know what you find.
