95 blazer - strange overheating problem

Overheating 95 Blazer..
Might be related to heater question..
95 4 door/v-6/auto/4x4 Cooling system has always been great.. Water pump started making noise a few
months ago so we had it changed, along with new hoses & thermostat..
It's been running like a champ, until today.. Took a trip to the States today.. Ran it at freeway speed fro about 150 miles, including long idle in line at border.. no problem..
Coming back tonight, the temp suddenly went up (at freeway speed) to make the check engine light come on by the time we pulled over.. Reservoir was empty but radiator wasn't hot! Popped the radiator cap and it looked a bit low, so we fired it up and added coolant until full. maybe 1/2 a gallon.. Temp went to normal, we took off and went like 1/2 a mile and the temp went back up..
Let it cool down, added a full gallon of water... temp went to normal.. stayed normal for about 40 or 50 miles and then started rising again... pulled over, let it cool down.. reservoir was full.. engine didn't feel very hot, I could rest my hand on the intake manifold for a second of two without getting burned..
Cooled it down, put another gallon of water in and it went to normal temp.. let it idle for 10 min, occasionally revving it a little, not sign of water leaks, radiator cap not letting off pressure, etc...
Got back on the road and went all the way home, about 70 miles of stop & go mixed with hiway speed.. temp stayed right at normal all the way..
WTF??
mac
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If the radiator is cool and the engine is hot sounds like a stuck thermostat. pull it out and put in boiling water to see if opens.
DZ
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Actually, the weird part is that when the engine was hot, so was the radiator, but then the problem seemed to come and go all night.. Very strange..
I checked it this morning and after the last run of about 60 miles last night, the water is almost full and no sign of over night leakage..
We're going to get a new cap (again) just in case and see if that makes a difference, or maybe the gremlin is gone..
mac
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