new coolant problem

i posted below but now things have changed. it runs great. its all good until the temp gets to just over half way mark.then the coolant comes back from the motor into the overflow tank, and gushes out.

i just put a new waterpump in it, tried 2 thermostats and checked and the radiator isnt clogged. i noticed theres a sensor on the thermostat housing whats that do? and what else could be wrong?

it also did this before i just put new head gaskets in, which is why i had to fix that i guess.