Maybe bad/worn alternator slip ring brushes? I can't explain why it
would sometimes come back on though in the air.

Our C-172 had essentially similar symptoms but it was the Cessna
overvoltage sensor that was tripping it off. We traced it to a bad
ground on a filter capacitor near the regulator, and the OV sensor was
tripping at an unusually low voltage. We finally drove the alternator
on the ground with a juryrigged electric motor (so we could work around
it) & used an oscilloscope to monitor the alternator output.

I'm not familiar though with Piper charging circuits.