When an alternator goes on full charge, its because the field is grounded. The only thought I have is that you when go to high revs, something in the alternator "flies out" and grounds the field. When the shop tested it it didn't rev high enough and did not have the vibration associated with being in the car. Coupled with their crunching the front of it and getting into the internals might point to the alternator is at fault. Just got a rebuilt one at Advance for $55 which may be a cheaper and less aggravating fix than going through ballast resistors, regulators, etc. My , I'm no expert. See how others weigh in.