REG50

I own a 2003 BMW 525i and had numerous check engine faults. Corrected all but had to replace the thermostat because I will be driving and the engine control would come on and the car would have reduced power and use lots of gas. When I stop and shut the engine off and restart, it was go a couple of miles and do it again. Recently replaced the thermostat with aftermarket, Its getting hot here in Ga and it happens even more now. Is it the aftermarket thermo or something else?