4.3l V6 Yamaha Stern Drive - Intermittent Surging at WOT?

I have a 1992 Chaparral 2000SL with a 4.3L v6 Yamaha (actually a GM engine). I am experiencing a surge sometimes when I open her up over 3500 RPM. The foot is properly trimmed, I have changed the water separater and fuel filter, and It doesn't do it all the time. Usually only after I run the boat, stop and swim, then run back to the marina. It bogs down and wont take throttle, but runs fine until you open the secondaries on the carb. Plugs and Wires appear to be ok, I figured I'd try the message boards before I buy new ones. Help!