I have a 1993 MustangGt with a 1991 Motor in it and have had alot of weird heater and temperature problems lately with it. The motor warms up ok but doesnt blow warm air inside the car? The temp will stay normal for a bit then will rise and drop randomly and the higher it gets the colder the air flow inside the car is and when the temp drops it start to blow warm air. I just put a new thermostat in it today and it seems to have helped out a little but the same problems are still there but not as bad. The temp has gotin high enough where I have had to pull over a few times other than that the car is very solid. If anyone has an idea on what could be causeing this please let me know,thankyou.

 

Make sure all the air in the cooling system has been purged. An air pocket will cause those issues. Also make sure the rad fan is working correctly.

 

your guage spikes because it's reading air temp. also why the heater doesn't blow hot air because theres no water in it. If your lucky there is air in the system. If your not so lucky you could be bleeding combustion gas into your cooling system thru the head gasket. Happened to me a couple times. Does it happen suddenly after mashing the throttle?
Jack the front of the car up and make sure that the radiator is the highest point in the cooling circuit then top it off. If that doesnt help then pull your plugs after it has cooled down and see if any of them are wet or colored different from the rest. The heater won't stop blowing hot air unless the core is filled with air or your damper suddenly switches positions.
P.S. It's full of air.