I'm having trouble with my 97 Mazda 626 2.0L. When it gets cold outside the car will overheat and the heater will blow only cold air. I can watch the temp. increase with the heater blowing cold air, then suddenly the temp. will start dropping and the heater will start blowing hot.
Sometimes it corrects itself and other times I have to stop driving.
Any ideas?

Check the thermostat. It may be sticking.

Try to check the heater control valve.