After you thaw all the frost, how long does it take to build back up to a noticeable level again? What tests have you done?

It sounds like all the usual suspects have been covered. I'd start looking at the unusual. The newer electronic controlled defrost systems are called adaptive defrost. They have replaced the old school timer because they are more efficient. They will only defrost after certain criteria have been met. They track several things, including evaporator temp and door openings. I have seen in the past where a faulty door switch will cause the problem you are having because as far as the control is concerned, the door is never open. Use your model number and look up a parts diagram for your unit. Find out if it has a thermister on the evaporator, or just a limit. If it has a thermister, I would test it to be sure it was working. If not, it's likely the door switch or wiring between the board and defrost system, thermister or door switch. Hope this helps!

So.... had another technician here, he's stumped. Ran tests and all looks good to him.
Question, is there a substance that runs through the coils? I cleaned up some goop a few months ago just under the heating element.
When I use the hairdryer to melt the frost (every three days now) it frosts up immediately when I plug it back in. It makes a whining sound for about 20 minutes, frosts up and then the whole unit seems to operate correctly for several hours. Then it will whine again, turn off, etc.
I was thinking maybe if there is a liquid of some sort that aids in the frosting perhaps it's all leaked out and the coils don't have the capacity of doing anything but frost up? Just reaching.....