Well that then sorta points to an intermittent power interruption.
If it's not in the Fusible link or bulkhead, I have seen loose connections on the back of the ammeter do that.
However; if the power supply remains solid, then I would test the pick up. Or maybe just do that first, cuz it's easier for sure than the other tests.
BTW. The two resistors in the Dual ballast block are not the same. That is why the connectors are keyed to only fit one way.
