Ford Repair: 98 ranger cooling system, blown head gasket, losing coolant

Question
QUESTION: My truck overheated. It was low on coolant. After adding coolant the temp gauge would rise normally but there wouldn't be much if any heat coming out of my heater then my temp gauge would shoot up almost to the redline then after 10-15 sec. it would drop to normal and then I had heat. I changed my thermostat and when doing so realized that I was losing coolant out my exhaust so I put stopleak in and that stopped it (I think) but after putting in a new thermostat I still have the same problem with the engine temp shooting up. Can you tell me whats going on?
thanks

ANSWER: Hello Josh, sounds like a head gasket has failed. Sorry !! Thanks Tony   Sorry Josh, when the gage fluctuates it is because of air pockets that interrupt the flow of coolant to properly cool  Thanks Tony

---------- FOLLOW-UP ----------

QUESTION: thanks for the answer but why would that make the temp shoot up from normal and then drop right back to normal just as fast.  
         Thanks

Answer
Hello Josh, when the flow is interrupted it can not cool so temp rises. The hot water/coolant is cooled down by air flow across the radiator fins when the water/coolant passes through the radiator, no flow no cool. with a blown head gasket it causes the flow to be interrupted.  Does this help ?  Thanks Tony