wow that's pretty screwy.
Not overly familiar with the way the field coil wiring is done, is the field coil ground common to the rest of the harness or run separately all the way back to the PCM? I would put the voltmeter on the connector at the PCM and wait for it to flake out, see if it shows a problem... if it does, move it one step downstream, continue till you find the bad spot.
Yeah, intermittent wiring faults are about the most frustrating problem there is to debug. Intermittent software/firmware issues are the only worse thing, as they can completely blow away any trace of info that you could use to find them... at least wiring and hardware stay in the same place usually.