'98 Outback Sport: Electronic Door Lock Problem

The electronic door locks on my 1998 Subaru Outback Sport wagon have decided to go flaky in the last couple of weeks. Sometimes they work; sometimes they work partially; sometimes they don't work at all and I have to lock all the doors by hand. That isn't a safety risk, but it is a nuisance.

Is this a common problem? Has anyone here seen it on other Subarus? Any idea what is causing it, or how to fix it?

Thanks! :)