My Power Shower runs hot & Cold?

I have a TritonAS500 and a T550i pump, I am always replacing the thermostat as it runs hot and cold, is this either not a reliable shower or the fact that the thermostat i am using is LP (low pressure) and i don't know how to tell if it should be an LP or an HP thermostat. I am at a loss as to whether to replce the whole thing or not, it's 14 years old.

Colette
January 2011