1985 745t 256k miles

This is a long post, but its a REALLY wierd problem that I can't track down, so I figured the more information I supply the better. If anyone can solve this or offer insight I will be amazed.

Replaced plugs with new, gapped ones and drove to store yesterday afternoon. Came out, started car, tach started jumping around from as low as about 300 to as high as about 1900. Finally settled down and drove to other store, but problem kept cropping up as I was driving. When it happened car would buck with the tach swings, kind of lurching forward.

Since then I have tested it thoroughly. Sometimes it starts, sometimes it doesn't. Often when it DOES start the first time I try (like in the morning) it will idle for 30 seconds, then die. Then it won't restart for awhile, and then it might restart and idle PERFECTLY totally out of the blue.

The wierdest instance was last night when I was testing. I turned off ignition and the tach CONTINUED to bounce around, going from zero all the way up to 1800 or so, then back to zero. While it was doing this the Fuel Pump Relay was clicking almost in tandem, like it was going on and off.

I checked all the wiring, harnesses, etc. and can't find an obvious fault. The wires on the pump look good and jostling them does nothing. Jostling and playing with the bundles in the engine compartment also does nothing; if its running right it will continue to idle fine no matter what I do. This includes the plug wires. Also, jostling the relays and teh relay board has no effect either.

The tach oddity convinces me it has to be some kind of wierd short or grounding problem, but maybe I'm totally missing an obvious fault. The trouble is that I can't find ANYTHING that could be responsible, and my harness is brand new (installed in January).

Thanks for any help. I'm supposed to head out on a 2400 mile trip tomorrow and I don't want to take the Magical Mystery Brick on the road without knowing the source of this fault.

Best,
rt