I just finished similar problem with my 3.7L. It kept running between normal and hot. After changing stat, pump, sensor, cap, flushed with commercial cleaner but when I refilled with coolant I back filled it through top engine hose and It took about 2 quarts more coolant. Runs fine now. I suspect I had air in system.

 

I would have advised the OP to "burp" his system, but assumed if he had it to a shop they would have ruled air in the system out first.

But you know what they say about ASSUME. So maybe you just need to burp it. If you've never done it, there is a DIY in the FAQ section...