Try the following, make up a temporary wire. disconnect the VR IGN connection, and connect it direct to battery (or at starter relay "big stud) temporarily. See if this tames the surge.
if not stab a meter probe onto the battery NEG post and the other into the mounting flange of the VR. You want very low, lower the better, zero volts is perfect. If not, the grounding needs improvement.
Do that measurement with engine running fast to simulate "low to medium cruise" and first with all accessories off, and again with lights heater, whatever loads you can bring on powered on.
If this does NOT seem to be the problem, have a look at the alternator brushes and slip rings. Worn, dirty/ greasy, gummed up, stuck, etc
If that is not the case replace the VR
