Coolant boiling in reservoir Options

[brandon702bmx]

I have a problem that I've noticed recently with my 1994 Celica GT. I did an engine swap recently and I've noticed something I've never had in the previous engine. I put another 5SFE in by the way. I redid the water pump and redid the gasket for the thermostat. When I run the car, for a period of time and shut it off, I've noticed a bubbling that under the hood coming from the coolant reservoir. Can anyone give me a culprit or a common issue with this? I've looked it up other places it says a few things: bad radiator cap, improperly sealed head gasket. I'm not really sure where else I should look? Sometimes, not very often, the car will overheat will driving and if I put it back in neutral and rev the engine it goes to normal operating temp. The temp gauge goes to about half way where it should be. I'm lost. Can anyone explain where I should start looking?

[Smaay]

its not the thermostat. like mentioned if the engine is cooling itself then the thermostat is opening. Id be more looking at blocked water passages in the block. Its iron and you dont know its history.