NS30A2 issue maintaining RPMs

I recently acquired a 1986 model NS30A2 with a jet lower unit. It starts, idles, and accelerates nicely. When operating at full power with the throttle wide open, the RPMs regularly dip down to what feels like 2/3 throttle. This is while maintaining wide open throttle. The RPMs will remain lower for about 10-15 seconds, then it picks back up to normal for a minute or so, then the cycle repeats itself as long as you are operating at WOT. It doesn't stumble or cough, it just feels like you're backing off the throttle to 2/3, then opening it back up, while in reality the throttle blade is staying wide open (I ran it with the cowling off to verify this). Any ideas of leads would be much appreciated!

When was the last time the carb was Properly cleaned?