I know this is a long shot but my port motor is going up and down on RPM'S while underway. Running the motor at 4000 rpm then drops to 3000 rpm and back to 4000 rpm. motor is not cutting out and the tach is working . Just checking that someone may have had the same issue and can point me in the right direction.The motors 575 hp with carbs.

if you have blowers its called blower surge which is common @ idle..someone probably fiddled with the carbs,time to possible have them cleaned and serviced..

You really shouldn't get blower surge at 3-4000 RPMS. You need to check fuel pressure AS SOON AS POSSIBLE. It sounds like you may be starving that engine for fuel and if a blower motor runs lean it will be TOAST in no time. If you don't find a low pressure issue it could be in the carb. Don't run the engine till you get it figured out though as major damage can occur running under boost with a lean mixture. One other thing I have seen. If the engine doesn't really seem to be losing all those RPMs and the tach is just moving up and down, it could be the distributor shaft loose, check that and for evidence of oil in there, this will cause the tach to go screwy.