I am currently working on a 2wd 1992 Toyota pickup. It is having some strange over heating issues. I have replaced the thermostat, cold temp sensor, cold temp sending sensor, radiator is full and there is a slight power cut out on acceleration and it is running a little rich. Timing is set correctly and all new tune up parts. MAF tests good and cold start injector was replaced..

Here is the basic thing it does.. I start the truck and turn the heater on and after I drive about 2 miles the temp gauge will climb to almost hot. Then it immediately drops to normal temp. If the heater is not turned on, the temperature rises to the normal range and stays there.

 

Common issue, mine did the same thing, see this http://www.4crawler.com/4x4/CheapTri...tml#Thermostat

 

Concur.