Very werid A/C problem

[AgentJean]

I have a 95 Nissan Altima; this car features a climate control system.
When I first start the car the A/C is blowing out cold air within 5 minutes of engine turn on(temp is around 50 degrees) after driving for a while the Air coming out of the vents will jump to about 90 degrees for no apparent reason(I leave the temperature set a 65)

I've been testing the R134a pressure every now so often and for the most part it is within acceptable ranges (25-45lbs) during the tests the A/C is working and blowing out cold air. Today I had a chance to test the pressure while it was acting up(spiking to 85 degree air after putting out cold air for a while) the pressure red lined at 100PSI :Q
WTH is going on here? What could cause the pressure to spike like that after it was normal earlier in the day (I never added any coolant). I have taken the car to a shop and they said they can't find anything wrong(of course, cars always act normal when the mechanic is around)

Would venting the system, pulling a vaccum and refilling it with coolant fix this or am I looking at something more complicated here?(Of couse I'd have a shop do this. I don't have a recovery unit)

My A/C repair knowledge is very limited. What I do know I picked up from my old man when I was a kid going on service calls with him and that was only home units.

 

[hoorah]

The static pressure of an A/C system is about 100psi. If you're reading 100psi on the low pressure side of the system, it means the compressor is off.

Chances are you are just low on refrigerant. The compressor shuts itself off if the pressure gets too low.

Coolant is not the same as refrigerant. Adding coolant to the system will not change anything. If you feel inclined enough to add refrigerant to the system, just buy a can and follow the directions.