I don't know if anyone has mentioned that the '86 model still has the poor wiring harness. The insulation degrades and strange electrical problem can result. On several of my 240s, including my '86 245, I replaced the electrical wires (2) which go down under the front of the engine. On the '86 I ran the red and black wires back along the passenger side fender then the firewall to the gray connector. I spliced them in the and cut the original wires loose. The rest of the harness still is working ok.

The problem you mention sound like the harness may be at fault.

When I get that white residue on the battery terminals I clean them up with a solution of baking soda,rinse, dry, and coat with a battery protective substance or silicone grease. I've also had stalling problems cause by a battery connector which was repaired by resoldering the connector (cheap fix).