Hi my name is XXXXX XXXXX I will be assisting you...lets start by getting a little more info
how frequently does this happen?
If I am understanding correctly ..the blower is still operating but the temperature just changes almost to a neutral position (not hot and not cold)
there are a few possibilities that may be causing your problem..
2. the blend door actuator...this is attached to the heater plenum underneath behind the center of you dash
or three an electrical problem to either of these componets....the problem with checking a wiring problem is the problem has to be occuring to find the problem or everything will test normal..this is also true for the other components which makes diagnosis very difficult.
because of the age of the vehicle I would think it is more a problem with the switch or the actuator than a wiring problem as these things tend to wear out over time
and because it seems to default to a certain temp I would be looking at the switch if it were the actuator I dont think it would just change your temp ..if it were the actuator it would just not move and stay at whatever setting it is at. So I seem to have talked myself into a conclution that it is most likely a bad switch.
If you have any more questions please do not hesitate to ask