check magnetic clutch operation when u have hot air blowing. If not engaged, then faulty clutch. If engaged, check operation of the solenoid valves that control flow of coolant into the heater core.(they probably do not close fully there by compromising cooling efficiency).
TEST #2: If u have warm air blowing feel the larger of the a/c pipes at the firewall.. If cold, then it's likely the solenoids. If the larger pipe is warm to hot. A/C problem. Suspect expansion valve or partially clogged system.
Other factors causing loss of cold(intermittently):
-too much oil in system
-moisture in system