About a month ago or so while I was driving in the winter with my heat on my mazda B3000 check engine light came on, the heat went cold, and the gage showed that it was over heating as I slowed down to pull over the gage went back in the middle and everything was fine. Today a similar thing happen. I was driving with no heat on and the check engine light came on and the gage went up to max overheating and after a few seconds it went back to the middle and everything was fine. Could this be because the thermostat is getting stuck? Or is it a bigger problem like the water pump?
Thanks for your help

 

Since it is reacting so fast it is more likely the thermostat than the pump. Being this old I would give it a good cooling system flush, reverse flush the heater core and replace the thermostat with a good quality piece.

 

I'd make sure that the entire cooling system is purged of air. Maybe jack up the front end and run for 15 minutes with the radiator cap off, topping up as needed.