Out of nowhere, on my way home from work I saw the engine was overheating, when I'd press on the gas it seemed to make it climb, made it home, checked under the hood, and the reservoir was nearly full of coolant and it was bubbling.
I didn't drive it after that for a whole week, when I had time I ran it, checking for leaks, didn't see anything, I guess I didn't realise that it had overflown and caused dripping near the firewall, figured it could be the heater core, I'd had problems with heat before so I flushed it before, so I decided to check it out, it was fine, I didn't want to deal with putting it back yet so I bypassed it till I figure out what's wrong and ran it after to test, still overheats.
I took out the thermostat to test it, it seems to work, haven't put it back or done anything since then.
I replaced the water pump maybe 1.5 years ago and it seems to be ok, didn't really want to take it off.
I didn't notice any oil in coolant or coolant in oil
I honestly don't know what to do next, tried borrowing a head gasket test from the autopart store but they didn't have the liquid.
Is there any thing else I should do/check?

If it ends up being the head gasket, I know of a 00-05 (not sure of year) buick lesabre I may be able to steal the motor from, would it be able to fit?

 

You may want to check the Intake Manifold Gaskets, they usually go bad and need replaced and this seems to cause the overheating problem.
I have read many issues that have been solved by that fix. Sometimes it is the head gasket, but that usually shows a leak and maybe some unusual white smoke.

 

Honestly I'd just drop a new thermostat in there and take it for a test drive. They aren't that expensive, and you can eliminate that from the list of possibilities.

 

Check if your fans are kicking on? Maybe the fan relay is bad.

 

Make sure you bleed the air out of the system also.