The problem was way worse before I drilled holes in the thermostat. The bottom of the radiator was cool to the touch while the top was boiling hot. With holes in the thermostat it worked much better.If the water was flowing too fast it wouldn't have time for the heat to be removed in the radiator. That is what the thermostat is supposed to do though. Stop the water flow so the heat can be removed then allow it to circulate.
I think my next test is going to be a thermostat where I remove all the guts and just use it to seal the housing. That should prove whether it's low flow or too much flow causing the issue. Maybe I'll go pull the 187°F thermostat out of my old water pump and try it as well.