My ac works for a short time and then blows hot air only when it is over 85 degrees outside. I have done some research on line and thought that it was the thermal control amp and replaced that. No go. The system has been recharged, has adequate freon, has been tested for leaks and the compressor works. I am mechanically inclined but not a mechanic so I need some help troubleshooting this further. Step by step. I have taken it to two places and both said it was fixed. It wasn't. I am afraid to take it somewhere else because I know the drill and don't want to spend upwards of a thousand dollars on a car woth $3,500. Any ideas?

. Check/replace the A/C Clutch Cycling Switch:

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With a shrader-type valve, this should be very easy to do and not require evacuating/recharging the system.

. Check/replace the Thermostatic Expansion Valve (TXV):

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This requires evacuation/recharging. This is what I suspect is the problem.