502 MPI Low RPM Surge
 

I have '99 502 MPI's that are basically stock with Arizona Speed and Marine upgrade to the PCM's so I have to run 93 octane. The starboard motor is surging up 100 - 200 rpms when running slightly above idle at 800 - 1000 rpms. It may also do it when higher than 1000 rpms, I was going through a no wake zone so I don't know if it would do it at say 1200 rpms. It did not do it at idle and I didn't notice it while on plane.

Any ideas? Plugs maybe? Water in the fuel?