Shower becomes colder as it runs
Remodeled a bathroom a few months ago. Installed a brand new Price Pfister Pasadena single handle shower unit (installed by a professional plumber). When taking a shower the hot water starts out just fine and over the course of the shower (7-10 mins) the water gets colder and colder. Never totally cold, ends up lukewarm at best. We have to keep turning the valve more and more towards hot to keep it at the right temperature. That being said, you can go to any other faucet in the house and the water is still scalding hot. I've replaced the cartridge once so far and it changed nothing. I have Price Pfister sending me a new one again just so I can try it one more time. Any ideas on what else could be causing this would be greatly appreciated.
A bit more info in case it helps. The house is from the 1950's. I purchased it this year and the previous owners had all new copper pipe installed within the last 10 years so I think that is fine. The water heater (gas) is less than 10 years old. I've tried to turn up the water heater to the point that is almost all the way up as hot as it will go just to make it through one shower and it doesn't do anything except make all the other fixtures in the house way too hot. I've even tried turning it lower than usual to see if the anti-scald mechanism in the cartridge (if it has one, I'll admit I'm not sure) was getting triggered by the hot water being too hot coming into it. Nothing seems to help so far.