When I shower why would the water change temperature by itself?

Hi LA Bijoux

The problem is with the mixing valve at the shower or it could even be at another faucet/valve.

Go around and shut off a hot water shutoff under every sink and at the washing machine (and any other places where you may have hot/cold together, like an outside spigot or shower). Now, try running the problematic shower again. If the problem is resolved then the problem is not at the shower valve and is to be found at one of the faucets you isolated.

To determine which faucet mixer cartridge is at fault, open one shutoff at a time and test the shower valve. When the shower valve acts up again, you will have found the faucet that is at issue and can replace the mixer cartridge.

If when shutting the shut offs off you see dripping at the handles, just tighten the packing nut behind the handle a little to stop the drips, OK?

Good luck!

Mark