2002 Chevy Venture overheating

Hi, I was overheating, losing coolant but no leak. Mechanic said it was the intake gasket, I had it changed. He showed me the old one and it was cracked. It was still overheating, took it back and he said he changed some kind of hose. Got it back today and $500 later and the heat gauge went all the way over to hot and the overheat lights came on, check coolant etc. then it went back to normal for a few miles and again by the time I got home the gauge went all the way over to the right. I'm thinking thermostat problem. Am I correct? Any suggestions? I don't really want to take it back to him, he had it for a week for the intake gasket and another week for whatever hose he replaced.