Following a 3 month downtime for maintenance, my Fountain 38EC, with 496HO, Starboard & Port engine seawater pressure indication is same until about 2500 RPM.
But around 2000 - 25000, the starboard gauge indication starts to rapidly fluctuate & bounce from 7psi to 15psi and never stops bouncing all the way up to WOT. And never gets as high as the port side. The port side is steady through the range 15 - 25 PSI..as I roll up to WOT
Both are merc pumps. Water & oil temps are good/same. Water flow out the exhausts all looks about the same. Have no codes using the merc scan tool. Both engines are 3 - 4 PSI at idle (using scan tool and on gauge). No alarms. I have hose fed gauges, I also bled the hose at the gauge. During the maintenance, the starboard engine was removed (replaced the merc transom) and reinstalled. I did not have this problems before the downtime. Any ideas?

Air leak on suction side of pump?

Swap gauge feeds and see if the moves. If it doesn't then agree might be a leak

Are you seeing water in the bilge too?

If so, most likely your air drain valves are poping open at higher RPM / (more pressure on the water side) the springs get weak over time. A lot of guys just remove them and put a simple piece of pipe in with a drain valve. If you search on here for leaking air drain valve there a quite a few threads on it.