I had a similiar problem during a recent trip to Vermont.  I needed gas and the
only place that was open was this little shack that was owned by a man with no
teeth and had a dog that look like it was 100 years old.  Anyway before buying
gas at this place my van was running real good.  After leaving the gas station
and getting on I-91 the trouble started.  I had symptoms similiar to what Derek
was talking about. I too have the replacement harness installed, and was totally
frustrated.  However, thinking back to that gas station and it's owner, I pulled
off the highway at the next exit and put in some high test gas, I only put in
$10.00 at the bad station.  The rest of the trip was trouble free.  My first
guess would be bad gas or possibly water in the gas.  

Mike Plickys
1987 Syncro