Voltage regulator contacts

I have a problem. Both of my voltage regulators seem to have failed
in an identical manner.

I have old-style three-relay regulators - one overvolt cutout, one
overamp cutout, and one contactor to connect the armature lead to the
bus. I can't replace these with solid state because none is available
- these are paralelling regulators, with a fourth tab that hooks into
a paralelling relay. I can't use them, either.

Seems that the contacts on the overvolt cutout have become degraded in
some way. Instead of essentially zero resistance (with closed
contacts) I'm seeing about 30 ohms. Cleaning the contacts with some
1000-grit sandpaper restores the zero resistance - and fixes the
generator problem, for a few minutes. Then the generators go offline
again, and upon testing, the resistance is back to where it was.

Anyone know a fix and/or the reason this happens?

Michael