Alternator and voltage regulator question

The "jump up when you get on it but not all the time" might indicate an erratic connection that is occuring due to the motor torquing/jumping around or just hard vibration. It is hard to say exactly where it is.

I would just start with the easiest, the ground side, and make sure all metal surfaces are clean and bright; and I would put No-Ox (or better yet, Cramolin) on the connections. Do you have more than one ground cable/strap from engine to chassis? Make sure you check the battery connection to chassis at the back too. Then move onto the hot side wiring and components.

I would connect a needle indicator voltmeter on the battery + and - by using 2 long test leads, and monitor that versus the VM in the car; you might see it jump on the car's VM and not on the test VM at the battery. Moving the test voltmeter's +connection point further and further towards the alternator might show the problem point.