When starting up, ALT light is on and VM shows discharge (about one needle width). After a while, revving the engine extinguishes it and that is mimicked by the VM. After a bit longer, it charges normally. If I let it sit for a day, repeats the behavior.

Checked the belt and it has 1/2” or less slack. Grounding out the VR gives me 16+/- volts at the battery cable. I installed a spare VR that I had (condition unknown) and got pretty much the same behavior. Cleaned the grounds, no change.

Suggestions?

Sound like the brush on the voltage regulator is barely touching, may be replace the brush assembly?

I believed when you rev the engine, the voltage from battery should be around 14volts only not around 16volts. Idle should be around 12volts.