From the original post that started this thread:

... underpowered - it pumps up
to about 95 psi ok, but then the motor stalls. The unloader bleeds the
pressure off, the motor restarts, runs for a couple of seconds, and
then stops again...repeat...repeat..."

Yes, those're the symptoms.

So it stalls first, then the unloader bleeds off pressure at the
compressor and allows it to restart?

I'm puzzled, but at this point I would disable the switch and unloader
and see if the compressor runs up to 120 as it used to.

You said the motor current is only 5 or 6 amps when it stalls?
Locked-rotor current for a 1/3HP motor should be much more than that,
at least 30 amps (I haven't looked it up). You're sure it's getting
full line voltage at that point? I don't see how it could be.

Alan