My boat is a 1985 Sea Ray 340 EC.  The hull is the tunnel drive configuration.  I understand that they don’t make them that way anymore because at some point you get very little increase in speed with a large increase in RPM.  I need to understand whether that is what I am experiencing or have a mechanical problem. It has twin Crusader 350 HP engines (454 cu in) and Velvet Drive transmissions. The port engine is counter rotating.  Both engines run strong and smooth and have about 650 hours on them.  Above about 3,200 RPM the tachometer for the starboard engine sometimes fluctuates upwards to around 4,200 RPM.  It keeps fluctuating rather than locking into a new RPM.  Other times the engines seem to stay in sync up to 3,600 RPM at a speed of about 30 MPH.  The synchronizer indicates it is running faster than the port engine.  If I back off slightly on the starboard throttle I start hearing a clacking noise.  My guess is that it is alternating between the engine driving the prop and the water rushing passed it driving the prop.  If I try to back off on the starboard throttle just a little it drops a few 10's of RPM but keeps fluctuating.  A little bit more and it drops to lower RPMs than the port engine and the boat slows down.  If instead I increase the starboard throttle a little nothing much changes.  With a bigger increase in throttle it will start to push the boat faster and with just enough throttle it sounds like the engines are in sync: no wa-wa sound.  At full throttle the port engine is in the low 4,000 range and the starboard is in the high 5,000 range and sounds like it.  (I am a computer guy, not a trained mechanic.)  It doesn’t feel like it is pulling to either side.This is my third season on the boat.  Based on the seller’s recommendation for the ideal cruising RPM (and being new to big boats with multiple engines) I never ran it beyond 3,200 RPM until well into the second season.  I had a good look at things between seasons and in the water a couple weeks ago.  (I’m a scuba diver.)  The props look like there isn’t a nick on either one.  The markings are hard to read: P27 and P28?  A friend looked up those numbers in a prop catalog and tells me that they are the same pitch and diameter but for opposite rotation.  Obstructions in the tunnels are the opposite of what you might expect.  There is a small transducer near the forward end of the starboard tunnel and a flush mounted water pickup a little aft of that.  The port side has a paddle wheel and a much larger transducer in about the same locations. Am I just experiencing the upper limit of the starboard tunnel and not the port tunnel?  Could this be a transmission problem?  A bad tach?  A prop that needs tuning?  An engine problem?