1. Leak in your A/C line, kaya biglang nawala ang lamig malamang ubos na ang freon. My primary suspect is your evaporator.

2. One probable cause is the vaccum switch which automatically adjusts rpm when compressor is charging. Another case is an IC inside your hood (which I forgot what that name is), that controls your engine's rpm if it's an EFI.

3. Sunong na motor, pinapalitan ng either surplus or brand new motor.

Dalhin mo sa mga aircon experts they know what to do, expect to shell out 3-5K for an evaporator, 800 freon refill, blower cleaning, fan motor is around 350 for surplus to 800 for a rebuild unit. And other plus plus...