Electric problem

I just bought a pick up truck chevy silverado 1993,130.000, v8
couple weeks ago and once and while when driving all gauges, battery, gas, temperature, etc. Start jumping up and down like crazy for 2 or 3 seconds then the truck just keeps running normal, then all gauges goes normal again, and if is night light gooes out, I checked battery connection, alternator is good any idea what could be the cause of this problem I apreciate any help.