On our last trip from Scituate to Nantucket my port engine would not rev up over 1500 under load. This occurred the first time after running for an hour at 1800 rpm then slowing to idle to close the forward enclosure window. After running for a few minutes at 1450, stopped and started from idle, redid sync, thinking it might be a synchronization issue. Ran fine the rest of the way. Some more residue on the transom than usual at the end of the run. On the way home it happened after we slowed down to 900 rpm to go through the canal. About 40 minutes of slow running. Port engine would not go over 1500 with sync or using individual throttles when we got through the canal and tried to get back to fast cruise. Both fuel vacuum gauges were the same about 9psi at 1480 rpm. No alarms and all the temps I could think of taking with the infrared were within a degree or two. Any thoughts on what to check while waiting for the MTU tech to find time in their calendar?