Here's a shot in the dark:
If it is a valve problem, I've replaced my valves in my shower faucet. However, I have a 2 faucet shower with separate hot/cold faucet taps so I have 2 valves in my system. But to replace them, it's fairly simple if you have the proper tool and slightly hard if you dont
What happened with mine is that the tabs on the valves shaved off or broke such that my hot water lever was overrotating instead of stopping. I'd be able to stop the hot water but not necessarily in the same clock position as before and if I kept turning, the hot water would turn on again. This lever is supposed to have positive stops when off and when full max which wasn't happeneing.
Once I took the handle off, there was a plastic lockring which you could remove with either a long nosed pin spanner. A spline tool you could buy from Moen, or slowly, via needlenose pliers. Being the cheap resourceful bastard I am, guess which method I got it off?
Anyway, once the lockring was off, the valve comes out and was replaced with a new one. Just for fun, I did the cold water valve too, because I'm not
that cheap.
Maybe this helps you look at your valve but I found Moen has a pretty good cut out on their website but perhaps if you know your mfgr, check that.
Could it be possible that your valve might of had the same problem, the tabs that keep the valve fixed might have worn off, allowing the valve to rotate... making your diverter's temp setting off.
Jay