’97 Pontiac Trans Sport – Car stalls in gear

1997 Pontiac Trans Sport 3.4L 6cyl, 110,000 miles
Repairs: Brand new cat and o2 sensors, new fuel filter 2 yrs ago, full coolant flush last week, personally cleaned MAF and throttle body two days ago, checked PCV (seems fine) wrapped broken vacuum line running from PCV with silicon tape. Poured Royal Purple fuel injector cleaner in a week ago. Spark plugs have been changed but not in past two years. Car primarily sat in garage for two years without use.

Symptoms: Car will start in the morning and can make one trip at least 10 miles. Upon restart, car will idle roughly and then stall out when put into gear. This has happened the past two days before taking it to mechanic.

Mechanic says the ECM wiring harness is to blame, determined by tapping on the wires and car stalls. The car would stall when I would tap on the vacuum line before I wrapped it. The harness is discontinued and they want to test and re-wire by hand, for a total of $800.

Any thoughts on what it could be? Should I take it somewhere else for a second opinion? What should I ask the mechanic?
Thanks.