Ampmeter flickering
Yup. This could be one of several problems. Could be a bad connection in / at the ammeter or bulkhead, or a problem elsewhere
Might be a bad regulator
Might be bad brushes in the alternator
Might be bad grounding between alternator (engine block), body, and battery
Or a poor connection "voltage drop" in the engine harness supplying ignition power to the voltage regulator.
What are you using for a regulator? Is this the old 69 / earlier mechanical regulator?
What year / model we workin' on here?