Engine chatter at idle when light load on port 8.1 engine

[2003380]

When I'm at idle on the port engine (650 rpm) and have starboard engine slightly higher (800 rpm) the port engine chatters as if it is misfiring. This only happens when the port tranny is engaged and stops if I put the port tranny in neutral or increase rpms.
Mechanic changed a computer idle module and no change. Runs fine otherwise.

Mechanic says leave it alone.

Plugs, wires, coil?? What do y'all think

Thanks

2003 380 sundancers 8.1 gas

 

[87express]

My guess is that one of the cylindars is not firing. With a big block motor you may not notice one not firing until you put it in to gear. the chatering you are hearing is in the transmission due to the engine skipping. Most probably a bad spark plug or wire