Strange s10 problems..?

I have seen the speedo jump on several late 80's early 90's chevy trucks while reving the motor. This is when they first went to the electonic speedo. Not sure what causes that, Maybe in the instrument panel but did not affect the running condition. Check and make sure your distributor lock is in place and the distributor is flush with the intake, I have had them work loose and the distributor come up. Thus throwing off timing and moving the oil pump drive up to where it does not fuction properly. If it is up push it down and secure it. If it is not pushed up I would begin with plugs wires cap and rotor, but first inspect the magnets in your distribor under the rotor they can become rusted and cracked giving you similar problems. Best of luck