Shower faucet handles move when using diverter
I've got an old cast iron tub with a two handle faucet with a shower diverter. This is pretty old school so there's no pressure balance valve or anti-scald features at all. We've been having problems with the faucet handles moving (you can see them move) when we're using the shower diverter. The worst one is the cold water, so my wife can't seem to take a warm shower at all because when she turns down the cold water it gets immediately hot, but then the cold handle turns on its own again. What could be causing the handles to move and is there a way to fix the problem (that doesn't involve gutting the bathroom and replacing all of the plumbing)?