Speedometer jumping...why???

I have a 1998 5.7 Vortec in my Express conversion van and my speedometer just started jumpin around. It jumps from the actual speed down to 5 or 10 mph less. This also causes the cruise control to dis-engage. The transmission shifts smoothly and it doesn't jump between gears or anything, just a jumpy speedometer. Any ideas?
Thanks!

I have a 1998 5.7 Vortec in my Express conversion van and my speedometer just started jumpin around. It jumps from the actual speed down to 5 or 10 mph less. This also causes the cruise control to dis-engage. The transmission shifts smoothly and it doesn't jump between gears or anything, just a jumpy speedometer. Any ideas?
Thanks!

It could be the V.S.S. , But the first thing I would check is the positive battery cable. They sometimes corrode under the plastic boot and have been known to cause all sorts of electrical gremlims. Another thing to check are the chassis grounding straps that ground the chassis to the frame and engine block. I am sure there are other possible causes and these are just a few.

Yes I agree. My suburban was doing the same thing and it was the battery terminal. Just clean them up.