The engines are 1978 330's about 900 hours, log manifolds and thru hull exhaust, Merc transmission. Stock points ignition. Both engines start right up quickly and cleanly and idle at the same speed Engines are completely stock and have never been opened up from what I can see. I can be cruising at about 3000 rpms on both engines at a steady speed 1-2' chop. No change in drive trim angles or trim tab positions. The starboard engines will lose about 200-300 rpm slowly, which i can also see the gps speed drop 1-2 mph, no change in oil pressure, no weird noises. If I give it a little more throttle to get it back in sync, then speed goes back up to where it was before. Trans fluid is full. Outdrive fluid is fresh this year (TRS drives). Its a minor annoyance that I can compensate for now, I'm just looking for ideas on what might be happening to look at when the weather gets cooler and the boat is sitting more.

could just be throttle linkage, stretch, adjustment ?

Mark your sticks and see if it is possible that it is bumping down a little on it's own.