Welcome to the boards.
More clarification will help. You dont have a separate computer for the A/C. You are probably talking about the CCRM (a black box next to the radiator overflow bottle). It houses relays to control your fan.
If you paid to have this repaired, I would take it back because it sounds as though something was not done right.
Absent that, I would get the fan to turn on for no reason at idle. Then check for 12 volts at Pin 14 on that CCRM (Using your DMM, not a test light). And if the fan is on and there is not 12 volts at pin 14, see if Pin 17 shows ground (resistance).
That's where I would start, given the broad nature of the issue.
An alternative, assuming the fan motor is in good shape, is to install a stand alone fan controller (a Dc Control unit would work well) and circumvent the issue that way.
You could also install a manual fan switch to make sure the fan comes on. I would still seek a long-term solution, but that would work for a little while if you really watch the gauge.
Good luck.