My speedo did that, it would start freaking out, tell me I was going 120 when I was doing 50, etc. Occasionally it would go all the way around and come up against the 0 peg on the wrong side. Never fixed itself from that point, though, I had to pull out the peg. I fixed it by tossing the cluster and getting a new one.
My car's been doing something simmilar, i'll be driving around going about 60 and it will just start twitching, then drop to 0mph and sometimes stay, then everyonce in a while jump back up to whatever speed i'm goin.
I still dont know what the problem might be. It only started doing it after i got gears, i wonder if the different speedo effected anything.