[Slam]

Grounding problems are endemic with these cars at this age. You might want to spend an afternoon in the garage hunting down the grounding points and sanding the connectors. Look for brown wires. There's a ring of them under the dash behind the fuse block, these corrode. Since the dash wiring is in series, bad grounding can and does cause what you're describing. I usually go so far as to remove the dash and fab up a new ground harness from the back of the battery box as a redundancy measure. If you've got the Haynes manual it'll tell you where the major ground points are. Off the top of my head, look just behind the headlight buckets on the "frame" rails there, the aforementioned ring under the dash, the rear of the car near or between the tail lights and anywhere else you find a brown wire skulking. Of course, a biggie is the battery ground to the bellhousing. In many cases replacing this cable is the best way to go. I added a strap from the battery to the intake manifold too.

Have fun!