Need help! coolant overflow issue

Need some opinions help on the following problem:

Coolant leaking top of timing chain cover - so I replaced timing chain cover
still had to keep adding coolant. Block is dry all the way around engine except for driver side rear - wet under head - probably head gasket. Car runs great no smoke out the pipes.

New (and main problem) - Past two days started car up to idle in the am while I get the kid ready for school. Come out probably 10min. and Stewart Warner temp gauge reads 220deg. I open the hood and coolant overflow is full and overflowing.
yesterday I blipped the throttle and temp dropped. I can drive around all day and no problems. Has only done it twice in the am when first start up. Thought about thermstat or water pump. If all the coolant is being held in the radiator then doesn't it seem llike the pump is not pumping???

Any help would be greatly appreciated

Thanks

Thanks J for the in depth info. I usually do not have the time to do major stuff myself, Sometimes it's worth paying someone else so I don't have the headaches. Like "Oh **** I broke a bolt" and you look around and find you don't have a second car to go get another one. I will add your info to my tech list though

The 331 motor was put in last Oct. 10:1 compression Talked to my shop today and they said it may be the head gasket and or the head bolt. The bolt is wet and the coolant may be running down the block making it look like a head gasket leak.

The other issue with the coolant overflowing. Yes I agree it looks like the thermostat is sticking. I bought another and went to replace today, but discovered that my newly installed aftermarket timing chain cover flange is slightly thicker than the original stock one, just barely (by a **** hair) keeping me from extracting the one bolt. I dremeled the timing chain cover to get it out to no avail. The bolt is going to have to be cut and a new shorter one put in. See first paragraph.

Anyway I bolted it all back up, filled up the radiator and sure enough it started to overflow. I moved the car back and forth in my driveway and bam, temp went down and thermostat opened. Sooooo it looks like a date with my shop Tue and to do compression test to verify the head gasket leak.

Thanks everyone for their 2 cents. This is a great site. with a lot of info

93 Mustang coupe, 331 Stroker, 10:1 Comp. Eagle crank & rods, SRP pistons, AFR 185 Heads w/HD valve springs, 1.7 Rockers, 24lb inj. 190lph Pump, Cobra Intake, 70mm TB, 76mm MAF, MSD Ignition and coil, Pulleys, Canton 7qt pan, Lakewood struts and shocks, Megabite Jr. Lowers, BBK HD uppers, Sub Frames, no sway bar, Mach 1 Hood, and battery in the trunk and a few other parts.

Well looks like it was the thermostat sticking. Did a pressure test and no leaks from heads, gaskets, bolts etc. will see how it runs tomorrow

Thanks for all the info and suggestions