Port Engine RPM Issue

[hd2002hd]

1998 Searay 400 with 7.4’s. When coming off plane the port engine will roll the RPM’s down to about 600 as it should, but the. It will rev a little on its own and fluctuate between 1000-1100 rpm’s. Not every time, but I would say the majority of the time. If I use the throttle to rev it a little it will usually come down to normal. The starboard engine doesn’t do this. It does have an engine sync and although the starboard is around 600 rpm's I wonder if that affected it, ur sync switch is off. Not long ago I had to put a new distributor in the starboard side so I changed plugs, wires and distributor cap on both engines. However, it did this prior to that work too. Any idea what would cause the increased rpm’s?

 

[Island Time Buffalo]

My issue is more general, but similar. The engines both often waver up and down at idle and low RPM (not just coming off plane) by as much as a couple hundred. My mechanic is convinced its the IAC valves - says they are notorious for this. But I have the retrofit risers after the 7.4 water ingestion issue, so I have to basically take the top of the engine apart, so it's an off-season repair.