The one that's been in the news most lately for technical problems is Spirit. It's been failing to wake up when scheduled on a few occasions, and it's also had instances of "amnesia". My guess is that the NVRAM is starting to wear out; with our flash cards and things we tend to think of such memory as being practically immortal, but it's not really. It does wear out eventually, and if you're using it *really* hard, like the MERs are, that will come sooner.
I remember one of our test units here (not related to space at all, but using NVRAM) wore out its NVRAM a few years ago. We'd been using it for software stress testing, software regression testing, and development work, so we were hitting the NVRAM a heck of a lot harder than it would be in the field. I think it was about as old as the MERs at the time. Only a small part of it wore out, but we replaced the whole thing. Obviously, the MER team doesn't have that option; if NVRAM really is failing, they'll have to find clever ways of using RAM to fill the same function, which will limit the amount of data they can transmit home.
Still, Spirit's been driving again, so they're keeping it going! Those MERs never cease to amaze me.