I recently put in a Grohe combination tub filler, hand shower temperature controlled valve assembly. It is primarily all brass (probably weighs 10 pounds) and was not cheap. I'm sure other companies also make things that would work. I think you need to try to figure out what is causing the temperature flucuations, though. It might be that just adding a pressure balance valve is needed, or if yours already is, it is sticking. It probably can be repaired. My unprofessional opinion.