It could be a combination of both bad grounding and a failing alternator. In that case your battery is not getting fully charged. You need to look around the engine compartment ( fire wall and wheel wells) for any grounding studs and wires. On your age vehicle I would bet they are all oxidized. Wire brush them and open and retighten the nuts a few times..... and it would not hurt to coat them with dielectric grease (AutoZone) . Also check or have your alternator voltage output checked. It should be anywhere from 12.5 to 14 volts at idle. If the brushes are worn you might get that at speed but when you idle around in traffic the output could drop to almost ZERO.

Good Luck

__________________
FRED

Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K