You sure the thermostat is working OK? Blown gaskets usually don't overheat that fsat. The ones I have seen, just blow coolant out the overflow and once it gets below the sender, the temp gauge reads cool or normal ( sending units will not read steam). Cracks in the head can blow coolant out like above, or suck coolant in and burn it, in that case, you can smell the exhaust. It will be kinda of sweet smelling.
If the exhaust is leaking into the coolant, that could it heat up really quick, but I would think that should blow coolant as well. You might really look into the exhaust ports for signs of something, anything that looks odd I would be leery of. Exhaust pressure in the ports ( after the valve ) is somewhat high for a short burst, nothing like compression in the cylinders. Dad had a Pontiac head rust out from the inside into the intake port. It didn't look like anything but a small white line till we poked at it. It turned in to a very thin walled 1/4" wide hole.
Joe
