1985 745t

Car previously would sometimes bottom out on the tach when first in gear, but no major problems. Past weekend car started having wierd problems starting (would start sometimes, often wouldn't), idling (tach would sometimes jump around while driving, often while idling), and just generally running.

Basic shadetree skills led me to conclude it was something in the ignition system; no start included no spark (I think) or fuel (dry as a bone when cranking), turned over strong but no response. I figured it was likely the hall sensor as that seems to be a common fault, but lacking any real skills I couldn't diagnose it for sure.

Took it to my indy guy for more thorough analysis. Car of course only failed briefly at his garage ($@#$@#$, too briefly for pinpoint diagnosis), but he tested the ICU and other items and concluded it was either the Ignition Coil or the Hall Sensor. He also pulled the coil when it wouldn't start, replaced with test coil, car started, he replaced old coiul, car AGAIN STARTED FINE!

My question is how can I tell for sure which of these damned parts is having the intermittent problem? I don't want to replace the Hall Sensor/Dist (I'm not equipped to rebuild the Dist. and would rather go new) unless I have to, and I have to go on a trip asap. Is there an easy or reasonably easy way to figure out which component is responsible for an intermittent failure? Can someone walk me through a test?

At this point I'll probably replace the coil and try and pick up a used Dist. for a back-up on the road. However, I really don't want to find myself in the middle nowhere pulling a distributor on the side of the highway.

Thanks,
Rick