I may be fighting multiple problems but this one has been a bear.
87 740 GLE with 363K miles.
In January (in Ohio) I opened the wire harness under intake manifold because I had a miss when I hit a bump. Repaired 6 to 8 bad wires. On test drive car went about 4 miles and quit at a stop sign. Replaced power stage and could start but sounded like timing way off. Local mechanic replaced "bad" distributor. Ran good, but noticed tach would jump 500 to 1000 rpm with no pattern when I got it back. Engine speed did not actually change. Jump eventually went away. When outside temp got warmer I noticed the misfire and loss of power above 2200rpm. The warmer it got the quicker the problem occured. 50 miles in March, 1 mile in July. 20 miles now in October. I noticed honking the horn caused full scale movement of the tach and affected the misfire. Not really better, but changed. The left indicator also causes tach to slightly bounce. It began running real rich but air mass replacement solved this (related?). I changed ignition switch and checked wiring harness with left indicator, horn, and distributor wire in it with no results. Have also reopened harness below intake and all is well. I have also replaced coil. Also noticed a discolored ground wire from engine to firewall which would indicate at some time it got real hot, possibly from what I think was a major short which started this problem. Any ideas? Thanks.