I have an '86 Dakota. The heater sometimes blows hot air and will then start putting out cold air. The temp guage doesn't show that the engine is running hot. Could this be the thermostat?

 

You might try replacing the thermostat, and also flushing out the cooling system (engine block, radiator, and heater core). Instructions for the heater core are found here, its not an exact match, but should loosly apply to your truck.

 

i recently changed the thermostat because the gage wasnt rising high enough. also changed the radiator cap as the system wasnt presurizing. change both, they are cheap parts and do the core flush, that fixed it for me

 

It could be, but more likely your engine's coolant level is a bit on the low side, as failing heat is a tell-tale sign of low coolant levels.

Check it only when the engine is cold, like after it has been sitting overnight, uncap the radiator and top it off, see if that helps.
You may have to repeat the procedure, and also start looking for leaks (just in case).

 

yes replace the thermostat, i had a problem like that.[/align]and lowcoolant does not cause failing heat it causes more heat, less coolant means less liquid to cool the engine, so it will run hotter[/align]