You say the fans are coming on, they are signalled to do so by the CHT (AKA) CTS sensor, so it could be this causing the problem as well as the stalling problem, assuming, of course, it isn't actually overheating.

Other causes could be air leak, or dicky ISCV or VSS.

Re the fan problem, resistor pack, very common problem, reason it works on position 4 is because it has a direct live for it.

A/C sounds like a idle regulation problem, try applying throttle and then turn on and see if it still stalls. If not then the ECU is either not recieveing the correct input, or not giving the correct output for air compensation to stop to raise the idle speed to stop this from happening.

Martin