We know that the compressor clutch is getting voltage. But then the voltage turns off prematurely, or it doesn't turn off at all and frosts the suction line (and probably the evaporator). We've covered a number of other possibilities, including the relay, which has been replaced with a new one.
All of this still tends to point toward a faulty thermistor, even though it was changed once already. When you try another one, let us know whether that fixes it. If not, double-check to ensure that the probe is positioned very close to the coils on the correct side of the evaporator and that nothing is insulating it from getting a good temperature reading. Sometimes, thermistors are shipped from the factory with a plastic protector on the probe's tip that's easy to overlook. It should be removed before installation.
You didn't mention the ambient temperature at which the pressures were recorded. But these readings appear to be elevated. Once the compressor is running properly, re-record the ambient temperature and the HP/LP readings at around 1,800 RPM and post them here.JohnHere wrote:The ac pressures at idle with blower at max and windows open are: 45 psi low and 260 psi high. Unfortunately we didnt get any readings at above idle speed.