fab,

I think it would be worthwhile to do some searching and find the thread that tells how to use an LED to do a diagnostic check of the climate control system and see if there are any trouble codes set. There is a connector near the battery next to the heater valve that will blink a trouble code for the subsystems such as climate control, srs, abs, etc.

If it blinks a code then there is probably a temp sensor out and you can decipher the code to find out which one.

Since it blows cold and THEN hot, it's not the refrigeration system giving trouble it is something to do with the climate control.

Good luck,