I have a Dereve water governor connected to the cold water pipe that goes to my mira shower. I can't find anything on the net about it or what it does. But on occassions my water is too cold in the shower so i have to turn up the heat, which then trips out my boiler, there looks like there is a screwing implement on the water governor is possible to adjust it? or has anybody ever heard of one??!
From what you describe what you have is a pressure-reducing valve and I would doubt that it would cause those symptoms. It sounds more likely to be a problem with the boiler. Is it a combi boiler? There could be a blocked filter in the inlet pipe, or a scaled up heat exchanger.