This is not caused by a pressure drop as far as I can tell, flushing the toilet doesn't affect it and it happens with no other water usage. it can happen 10-15 minutes into a shower, not just at the start. I've replaced the cartridge and the shower head. The water heater and pressure valve coming into the house are new, the problem seemed to appear some months after they were installed.
I'm looking for any and all suggestions I even bought a shower head with a thermometer to track it.
Best Answer
I finally figured it out, the problem was different from what I thought. The real problem is that 3 of the 4 RP19804 I tried were defective. You could not adjust the temperature in small increments, any small change caused wild temperature swings. Putting a handle instead of a knobs helped a little, but the real answer was to keep trying valves until I found one that worked.