My Corolla is a 1996 Toyota Corolla Gli, 1.6L, Manual Transmission, EFI.

I have been having problems with my A/C system. There appears to be no pattern on when this is occuring. Whether it's a cold or hot day, fan in the highest or lowest setting, temperature control in max or min setting etc., the A/C just erratically quits working. I've noticed that sometimes, the compressor clutch engages and sometimes, it doesn't. So I did the following to trace the possible cause of the issue:
-There is enough refrigerant. Minimal bubbles on filter drier sight glass.
-Clutch is functional. If I directly energize it or short the relay contacts, the clutch would engage and AC works.
- Compressor Relay has been replaced (just in case). No AC related blown fuses found.
- Checked the voltage across the relays. Whenever the issue happens, no 12V DC is being read.
- Disconnecting or shorting the pressure sensor does not help in engaging the compressor.
- Checked if the temperature setting (variable resistor) is functional. I got good readings from min to max setting.

What happens is, when I turn on the A/C, only the blower turns on. I also noticed that erratically, the engine's idling would go up but the compressor isn't simultaneously engaging. If I disconnect the A/C "blackbox" (under the dashboard), idling would get back to normal.

I am now suspecting that probably something is wrong with the "black box" that serves as the A/C controller. No observable burnt components there so I just cleaned the connectors with a contact cleaner. Until now, I am still getting the erratic A/C performance. What could be going on? How can I know if the black box is kaput? How about the thermostat? Could it be the culprit? What other sensors play a part in turning the compressor on? How do I proceed and troubleshoot?

Salamat sa mga makakatulong. Pinost ko na rin sa http://www.allexperts.com yan. Di pa nakakasagot yung mga Kano. Pinoy diskarte pa din inaasahan ko.