This is a problem we've dealt with but it's annoying and I want to fix it. We have a 15 year old (we've owned it for a couple of years), somewhat upscale for our area, 3.5 bath home with an issue I've never had before. When one shower is in use, and someone turns on water, flushes a toilet, etc., the water in the shower either goes colder or hotter. Is this a case of the builder using cheap valves, and they need to be replaced to fix the problem? We'll have to open walls to get at them, but I'm perfectly willing to do that if we can make this go away. Thanks for your help.