Update - bad news
It turned out to be a bad head gasket - a test confirmed exhaust gases in the coolant. Typically there is white smoke out of the exhaust with a blown head gasket, but for some reason even though I was going through a lot of coolant it didn't puff white smoke.
Big tip off - if the car is cold, and you have lots of pressure when you take off the reservoir cap its probably because of exhaust gases pressurizing the system. Also, with the car cold, take off the reservoir cap to release any pressure, start the car and run for 60 seconds, then screw the cap off - should not have a pressure build up.
Total job cost $4,400!!!!!!! and it wasn't at a dealer. There were a bunch of, "its your option, but as long as we are in there we should replace x". Also had the valves/head gone through $900 (part of the $4,400).
These cars are ridiculously expensive to fix, and not as reliable as some will lead you to believe. The car has a common 3rd to 2nd gear downshift issue. When the trans goes out it will be $3,000+.
I would not have spent the money to do the head gasket except the angle gear and rear differential had recently been replaced.
I am going back to GM.