Your mechanic is most likely correct with harness problem. I work in a dealer mopping up for Isuzu since they left the country and have had many of these.
The main harness (going into the PCM) is very often faulty with the wires breaking inside the insulation. It is usually where the harness bridges from the fender to the engine. Pull and wiggle this harness and see if the engine bucks or stalls. The normal issues with this is; check engine lite, trans light, hold light, bucking and stalling. Out of 20 or so of these harness problems I've only been able to fix 2. You can open the harness and check for broken wires and if your lucky you will find it. Most times however the harness needs to be replaced. Pretty costly I'm afraid.
There have also been PCM problems with these symptoms but harness is much more often.
Good luck and hope you find the problem.