Very much doubt its the thermostat, To rule this out, remove it! it is only there to allow the water warm up by circulating around the engine, then opens and allows the hot water to cool in via the radiator.
if your thermostat was stuck closed, it would heat up on normal driving.
Personally i think, with the boost your running, that your head gasket is passing under the load and heat involved.
keep us informed what the outcome is.
cheeRS
Ken