Your car is old enough to have the "rotten battery cable" problem. Cleaning the battery connections will not fix this problem. The wiring corrodes under the crimped connections and develops high resistance on the smaller of the + wires going to the dash. I cut about 1 inch off and re-terminated mine to fix the problem. I highly recommend doing this regardless of whether you have any other problems.

1. Fix the + wires by re-terminating.
2. Replace Ground cable (about $60 from Volvo)
3. Replace the alternator ground wire, and clean the terminals, they get oxidized a bit. The new ground wire is heavier guage than what they used to make. (at least on my 85, I don't know when they increased the wire size.)

R Duke