What would cause my speedometer to periodically not work?

I have a 1994 4x4 truck with a 3.0 eng. & manual transmission. My speedometer has a mind of its own & works periodically. Sometimes when it is working, it suddenly jumps to 70 mph. It also sometimes jumps & returns either to the original speed or falls back to zero, or when I use my turn signals when it's working properly, it will jump & return to speed I'm traveling. The odometer seems to work fine according to approximate mileage per gallon calculations.