2000 Mazda B3000 Cabplus temporarily overheating
 

So for over a year now my truck has been temporarily overheating when it is cold out. The gauge would go all the way too hot and go back down quickly. Last year I replaced the thermostat and got the system flushed because the antifreeze was like mud. By fall this year the antifreeze was like mud again and it would sometimes back up and shoot out of my overflow. A mechanic advised me to change the radiator and water pump because the impeller was probably corroded and when he took it apart it was. So I got both changed and got the system flushed again. On a cold day in December the overflow cap popped off and antifreeze came out while I was driving and the truck temporarily overheated for a second and then went back to normal. I changed the thermostat again. It seems as though it happens more so with my heater on, but this week has been in the 20s and it did it again for a second without the heater on. Now sometimes the gauge will go in the middle and sometime lower to the cold section for a little and fluctuate, but once in a while it will shoot up to hot for a second. Also the heater takes a long time to produce not very warm air but that may just be a different problem with the heater core. Do you think it is just air in the system?

Flush the whole system again.
I think your heater core and the lines to and from the heater are gunked up as well.
You could try disconnecting the heater hoses and back flushing the heater core through those. You could apply some quick squirts into the hoses with a pressure washer or at least with a garden hose with trigger spout.

When you refill the system make sure to get all air bubbles out. Watch some videos on how to do this.

Jack up the front of the truck. Take off the radiator cap. Start the engine, turn the heater on full too. Let it run like 20 minutes, continually topping off the radiator with 50/50 coolant as needed to keep full. After a while, the radiator/cooling system will not accept any more. Install the radiator cap. That's it.

Oh yeah: remove the jack before driving.