440 overheats once and then it's happy all day

The 440 in my dart likes to overheat once, and then it will stay 185 the rest of the day. It has a 180 tstat. When it gets up to 210 I can feel the thermostat housing, and it's not getting hot as it should. If I let it idle when I see it about to overheat it will hold 210-220 until the tstat opens for the first time. Once the tstat opens for the first time, everything is happy and it holds 185.
I have a 440 source water pump and water pump housing. I took it off suspecting that it was a typical 440 source piece of crap, but it looks like the ports are pretty clear and large. It is a driver side exit housing.

Anyone have any ideas? I have swapped tstats 4 times, same thing every time.