1995 Ford Windstar Coolant draining from radiator back into

Engine Cooling problem
1995 Ford Windstar 6 cyl Front Wheel Drive 129, 00 miles

I had the radiator replaced about 2 months ago(however prior I had used STOP LEAK for the old radiator). I also had the top radiator hose replaced and the thermostat originally for coolant leaking out before a crack was found in the radiator. After which the temp gauge stayed below the normal operating range even after driving car for a while. It would move upward slightly but still on the low end. Now I have noticed that after driving my car for about 1/2 hour or so the gauge shoots upward (has not hit the H yet) however my fans go on and when I looked under hood, the coolant level is very high and slightly spilling out the reservoir. I took it back to the mechanic and he now says it's appears to be a head gasket problem. I just want to know if all other problems have been ruled out (to be honest all I can take is their word). They said the coolant circulates but then there is alot of compression and the coolant is being pushed out into the reservioir. Could it be something else less costly?
1-Need new radiator cap?
2-Is Thermostat faulty or sticking?
3-Is the gauge set at the proper heat range?
4-Possible leaks in cooling system not detected?
Radiator clogged or partially clogged (due to the
STOP LEAK I had used)? I could still see some in
the coolant when the radiator was replaced. (I
am not sure if system was flushed properly)
I know this is a alot but I need to have a better understanding before I agree to costly repairs (or should I just start saving for another vechicle). PLEASE EDUCATE ME!