Almost certainly your problem is being caused by battery discharge. I've had a MkI for over a decade now; whenever the patches begin "warbling", I replace the batteries and everything is okay again (although if I've let it go long enough I'll need to reload the patches as well). It sounds like what you're encountering is the same thing. I'm not fully versed in the MkII, but try changing the batteries before you panic.
Oh - make sure you clean the battery contacts, too! A trick I use to get the inside one (at the back, where you can't reach) is just to open up a coat hanger and go fishing. I put a paper towel moistened with a small amount of Coke (it dissolves battery acid!)and follow that with a dry one. Even if you don't use the batteries for actually playing the synth, they support the memory RAM, and oxidation on the battery contacts can cut their expected life in half!
Hope this helps.