I have read where the ground bridge across the cluster can cause a myriad of problems. Pulling the cluster, cleaning contacts and adding a ground wire to the chassis can solve it. My '81 280 all of a sudden stopped charging and this was the problem.

A second thing I have experienced is that the older model fuses have alloy contacts that while they may look good the contact points can corrode (can't remember what it is called when two different metals are in contact, damn my old memory). Cleaning them and the copper contacts will be a PITA but might be worth a try. Also replacing with newer copper fuses might help.

These are the easiest fixes I can think of. Maybe others will chime in. Good luck.