Overheating

try a new thermostat and a coolant flush , let us know what happens

Already did that.

I had a van that did this, my radiator was going bad.

Radiator checked out fine. Have no clue to what it could be. Purged the line. Changed coolant resevoir cap.

Steven, have a combustion leak test done. if there are combustion gasses in your cooling system then you might have a bad head gasket. many shops can do this, or you can buy a kit from the auto parts store for about 50 bucks and do it yourself. lots of youtube videos on the net.

Can you just trust the Cadillac service mechanic to diagnose it?

Thanks Jim

Coolant reservoir was the problem on my 2000 Cadillac DeVille it was not the cap it was the entire reservoir tank on my other 2000 DeVille it was The Purge line stopped up everybody said it was head gaskets don't believe everything you hear try the simple stuff first