I just had my a c changed over to the new 134 type freeon. At times when driveing it quits blowing cold air and blows hot air. What causes that? And what do I need to do to fix the problem? My car is a 1983300D tubo.

 

My first thought is an overcharged system. Too much freon will coat the evaporator with ice and it will blow hot until the ice melts. Or a faulty expansion valve not regulating freon correctly. Or a faulty in car temp sensor. With the glove box liner removed you will see a hose going up to the small grille in the center of the dash just above the center A/C vents. If that hose is kinked, disconnected at either end or has fallen apart, the sensor cannot give an accurate temp reading to the programmer.