I have a couple of problems with my Xterra.

First my AC keeps cutting off and I have to power it off for about a minute before it will work again. I have already replaced several parts of the AC for various reasons. The condensor fan was dead so I replaced it. My high side line cracked at the joint and I replaced it as well as the receiver/dryer and I replaced the triple pressure switch while it was out. I also evacuated the system before recharging it. I had previuously replaced the thermo control amplifier which was short cycling the compressor constantly. Now I have replaced the expansion valve. If I remove the thermo control amplifer from the evaporator coil the compressor runs constantly and freezes up. Is the thermo control amplifier supposed to cycle the compressor on and off under normal conditions or is that done by the expansion valve and pressure switch?

My second issue is that the transmission does not engage in drive when cold but if I start in 2nd gear and shift up to drive after about 1/2mile it drives fine. It started doing this only when the temperature was below 40F but now it every morning.

Thank you in advance for any advice.