I had the intake manifold gasket replaced by a shop over a year ago, when I got the car back it immediately started to run very how once I took it out on the highway and had fluctuating heat. The shop that did the work said that it has an air pocket so I brought it back for them to bleed 3 times different times. Car still runs hot, they were getting frustrated and so was I. Then I decided to go to a different shop who, checked fans, radiator, bled the system with the bleed nut, changed the temp sensor, cap, new OEM thermostat & housing, and flushed heater core, car still ran very hot where I have to pull over before it actually overheats and still no heat. Shop then drilled tiny hole in thermostat and car runs perfect now, but I have no heat. They said it could be the head gasket, but oil and coolant are clean and no smoke from tail pipe. I've put about about 3000K miles since the hole was drilled and it's still running great, I just think it's strange that it all happened after the intake gasket was replaced and wouldn't it start to run bad if it truly was the head gasket? She has 84K miles on her.

Get a cheap chemical test kit and see if there are combustion byproducts in the coolant, that's the most reliable way to rule a head gasket in or out. If they had to drill a hole in the 'stat then it sounds like they had the burp hole out of position, if it's not between 10~2 o'clock then you can stand the car on the back bumper and the air still won't bleed.