My '05 has been inoperable for a while now, it started driving home from work one day, never had any issues before.
Since then it overheats immediately after warming up.
I had issues with the heater core so I decided to check it, it's fine.
Thermostat works.
Water pump is recently enough replaced/updated and seems to be working, didn't feel like removing to check.
No coolant leaks.
Doesn't seem to be coolant in oil from what I can tell.
Coolant reservoir bubbles.
Tried testing head gasket with tester but it filled with coolant after removing some.
Have replaced and bled coolant plenty of times.

About to either give up on it or take it to a mechanic, which I really don't want to have to do.

Any ideas?

 

How are you measuring the temp? Gauges go bad in these cars often..

 

It is probably the Intake manifold gasket, one of the frequent members had this issue a few years ago and had the upper and lower replace and it solved the problem.
Not sure what motor you have 3800 or 3400. Here are some Youtube links
https://www.google.com/search?q=yout...nt=firefox-b-1