90' 200 alternator flaky update

[Nick Lawrence]

Seems like it is temperature not rpms that make a difference,  never
faults when driving and moving, set in traffic a bit  and it will quit
charging.  No revving then will make it work.
Got stuck in traffic and it acted bad, when things got moving  all's
well, made a stop, drove home 15 miles, no issues.

Problem just reared it's head Saturday morning.   Alternator stops
charging suddenly,  usually happens while stitting still or at least
low rpm's.
Untill minutes ago it would come back when I revved the motor to 3k
rpm, and stay operational.

Does this sound like brush pack sticking?  Rebuilt alternator is about
5k miles old...but I didn't pull the brush pack to inspect.
Any advice?  TIA
Nick  central Ohio