Sounds familiar. Mine was doing about the same thing a few weeks ago, and I "fixed" it as follows:

http://www.peachparts.com/shopforum/showthread.php?t=131339

It's still working (about 1500 miles later), and I still don't know why it was off. I wouldn't recommend messing with it yourself unless you are willing to risk having to replace it. I figured I had nothing to lose, because it was pissing my off and I would have replaced it (or had it fixed) anyway. I assume I'll have to get it repaired correctly at some point, but in the mean time it's within 1-2 mph at 75.