Originally Posted by
kludgefudge
if it is actually "shifting" by itself, as in dropping to a lower cog of its own accord, it could easily just be that you need to tighten up the rear derailleur cable a bit. i know you say that you just had the bike tuned up, but that could very well be the issue if they replaced the cable, as cables can stretch, usually in the first little while after installing. if this is the case i would think your bike would be making a lot of unnessessary noise in a lot of gears due to the misaligned derailleur.
This is exactly it. If your rear derailleur isn't tensioned properly, it can "ghost shift."
Try unscrewing the barrel screw on your rear derailleur a turn or so to increase the cable tension. (It's the screw that the rear derailleur cable goes into.) Keep track of what you do, so you can undo it if you have to take it back in to the shop.