Speed-o-meter problems

I have a 1994, K1500 XC, 4X4, 5.7L, HD NV4500 MT, my speedo is starting to act funny. For about the last 3 months when ever I set my cruise control it will jump 10-15mph and when ever my truck slows down or speeds up with the cruise control it starts to jump around. Now every so often when I shift from 2nd to 3rd gear it will jump about 40mph then come back down to where it is suppose to be. Any suggestion on what this problem could be. I was wondering if it could be the vehicle speed sensor, but wouldn't that cause the problem all of the time?

Also wondering if anyone has put seats from a '96 or newer in an older truck, the drivers side on my seat is the 40 and the drivers side on the newer trucks is the 60.

Thanks in advance.

Horse482,
You usually give out good advice… I don’t know of an easy way to check this one. But if your truck was a 96 or newer you could use some type of data logging Scanner. Does anyone know of one that works on odbI? That would be the best scenario to eliminate your Sensor as the problem. How much is the sensor? It is worth doing the old part swap to see if it works?

I have been thinking about doing the part swap to see if that fixes the problem. I wish that it was a '97 or newer, the problem is trying to find on that has the hd 5 speed, obdII is a hell of a lot easier to trouble shoot problem, but then again obdII has it own problems. OBDI can be easier to trouble shoot, most of the time. Still hoping for advice on replacing the seats. Thanks