1994 GMC Sonoma SLE 4.3L erratic speedometer and Tach

Thanks for viewing my post. I recently purchased a 1994 sonoma SLE regular cab pickup, 4.3L "w" with automatic transmission. Sometimes the speedometer and tachometer perform normally but other times the speedo and tach go wacko. When it happens, both the speedo and tach will simulatneously bounce all over the place, indicating that I am traveling much faster than I am. The gauges are needle gauges, but I believe they are electronically controlled. I may be traveling about 40mph and the speedo will read 80 or 90 and the tach very high as well. I have been researching this and I believe that I may have a faulty Vehicle Speed Sensor. Any ideas? I am not getting any check engine lights or ABS lights coming on and all my other gauges are normal.
Best Regards,
Andrew :screwy:

In case anyone reads this, I have ordered a vehicle speed sensor (it was only 30$)and I will post later about whether or not it fixes the problem..

Update......
I found that I had a bad negative cable on my battery and this was the problem causing the gauges to go crazy. Although the terminal looked ok, the neg cable was internally corroded. To check for this test the flexability of your battery cable. When you bend the cable slightly it should feel relatively flexable, my bad cable was brittle and corroded inside which I could feel upon trying to flex the cable. I installed a new cable and the gauges have performed perfectly ever since. Too bad I already bought the Vehicle Speed sensor, at least if it ever fails I'll have one in my tool box :evillol: