The flickering is due to a flaky connection somewhere in the circuit or a load that is intermittent ( think turn signals as an example).

the ammeter is just responding to the current flowing through the system. the current is changing due to one of the following,
1. Ground issue between the body/ engine/ alternator/ regulator
2. Voltage sense line at the regulator is fluctuating due to
A. Ignition switch contacts /connections.
B. Bulk head connections.

3. Field circuit between the regulator and alternator has a flaky connection
4. loose /flakey connection between the battery and alternator which in turn causes the voltage to fluctuate in the circuit which causes the regulator/ alternator to respond.
5. Field / brush connections on the alternator are loose

You will have to try and isolate which circuit is fluctuating and causing the issue. refer to the previously posted article on how the system operates

NOTE
You will need a good analog voltmeter or oscilloscope as digital meters respond to slowly.