fluctuating ammeter

If you "bypassed the ammeter" and it didn't change anything, then you really didn't bypass the ammeter, that is if you mean the ammeter still flickers

But if you mean you bypassed the ammeter and the LIGHTS flicker, that is an indication that voltage is going up/ down.

Could be poor ignition voltage to the regulator or some other wiring problem

Could be bad ground at the regulator

Could be poor brushes/ poor brush connections

etc.