Warm Up Cooling Problem

I searched the general "car" catagory and the "Mustang" one without a match. I almost hope there is a similar problem discussed previously but want to apologize if that is the case for wasting anyone's time.

The car is a 95 6 Cyl with 107,000 miles. Was running great until I noticed the temp gage nearly pegged to the HOT side. Pulled over, popped hood but found no noticeable heat issue, engine seemed to be normal - warm to the touch with no hot smell. Heater was working fine, not extra hot. Started up and noticed temp gage went way down below normal, car was off only 2- 3 minutes. Once I drove a bit the gage headed to the very HOT side again and driving very easy I was able to nurse it home and kept the gage in reasonable area, (but still hotter than I liked). Once home checked coolant and decided to replace thermostat. Also checked gage sender resistance and that looked OK, 75 ohms cool and 20 when warmed up. Started car and noticed it took a very long time to warm up and then, per the gage, it went to Hot almost immediately. Have since changed out the thermostat, again, and the water pump. Old water pump looked fine and the change was likely a waste. Have flushed out the rad, the heater and the block but still have the same results. Why the long warm time? Maybe 10 minutes for the gage to move at all and then both the engine and the gage indicate very cool conditions. Things don't slowly get warmer. Why does it go to HOT right away? After about 15 Min. of running the upper hose, the bypass hose and the thermostat housing suddenly get HOT; real - don't even touch it - hot. If left to run after it gets HOT the temp gradually comes down to mid gage or even it will go to the very cool side and then it will suddenly go HOT again and the cycle will continue. A new wrinkle is that now the heater has quit working, why? The water passages all seem clear. Driving it seems to keep the engine on the HOT side unless I am very careful on the throttle. When refilling after working on the system I do remove the plug at the top of the engine so air can escape while refilling. What am I doing wrong? What am I overlooking? No fumes, white smoke, etc at the tailpipe. This car did have warped heads at the 30K mark and Ford did extensive warranty work at that time, (maybe 6 -7 yrs ago). The car starts and runs great, smooth no indication of the cooling problems per its running ability.

Sorry this is so long, sure appreciate your thoughts.

Regards, Skip

Follow-up - I will tow the car to the shop eventho it runs. There is general agreement we have a leaky head gasket so will be having those replaced. I did "burp" the cooling system several times and actually drove the car quite a distance and the temp gauge and heater worked, almost, normally. Now, I cannot get the heat from the heater and the car takes a loooonng time to warm and then goes to HOT (full deflection) on the gauge. Hope the mechanic has one of his better work weeks.

Regards, SKip

Yeah it's more than likely a blown head gasket. It's 8 out of 10 times it's the passenger side that goes. Matter of fact, that's what prompted the engine swap with with a 01 cobra engine.