Overheating Issues! Motor bad?

I drove my truck into town today and noticed that the temp was about 235 degrees when I was cruising on the highway. It has always stayed under 210 degrees until now. I played around with it a little and figured out that when I get in the throttle (build up boost) that the temp drops back down the normal operating range. About a month or two ago I had to replace piston number 7 due to running my motor lean (over boosting) and breaking the ring land. The valve seat on the head was beat up a pretty bad but I tested it for leaks and it didn't leak. My truck is starting to idle a little rough again like it did right when the ring land broke and every now and again it will smoke especially if i just let my truck sit and idle. Does anybody know what's going on here? Any help is greatly appreciated!

Also, could I be lucky enough for it to only be my thermostat going out??

Possibly a tstat but more likely a head gasket. Get a block tester.

Do a leak down test on all of the cylinders to test the head gasket.