I have a new 50 gal power vent water tank. I also have another 50 gal tank that is an over flow and heats my radiant floors. I am getting hot then cold then hot then cold water in my shower. I don't have an issue with sinks. Any idea what this could be?

What controls the flow to your radiant heat and is there a pump? Try lowering the radiant heat down several degrees and then turn on the shower, is the fluctuation gone?

Have a thermostat to control temp. I have it only on 59. And yes I have a pump. I replaced one tank because motor went and tank was 13yrs old. I had 2 ao smith tanks and replaced one with a bradford white

Did you try lowering the heat or turning off the power to the pump. I'd like to know if that stops the issue