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It sounds like the hall sensor, or connector to it, on the bottom of the distributor. It gets cracked and grounds out against the head. Each time, no pulses, hence no ignition, and no tach input. Then the engine shakes and it makes proper connection again. You'll probably need to buy a new distributor. I replaced this in our 88 740 last year with 180K miles. Cured all the problems. I even destroyed a catalytic converter with the raw fuel going out the exhaust... you don't want to do this.
It is POSSIBLE that you can insulate it better so it won't short out, but long term the thing needs to be replaced. Access back there is lousy at best so you'll probably need to pull the dist off the head to work on it. Two 10mm bolts hold it on. 8mm bolts are for the cap.
Good luck!
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Rob Bareiss, New London CT ::: '87 244DL/M47- 221K, 88 744GLE- 202K, 91 244 181K, 88 244GL 145K
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