thermostat makes it overheat?

[71GMC_3/4T]

It's also a good idea to run 50/50 antifreeze & water. That will keep it from boiling too.
I agree with everyone else on the little hole idea. Air will get through that little hole real fast and go to the top of the radiator where it belongs. When the T-stat's closed, there won't be enough water getting through the little hole to make a difference.
And put some clothes on your girlfriend! No, wait, I think I see where you're going with this... "Whew, sure is warm in here! Maybe you oughta take that fluffy sweater off... There, that's much better!"