Is it doing this when you actually have it throttled up? In other words, is it actually just losing RPM and then gaining it back? If so, then you likely have a fuel starvation issue like I recently had with my 6405. Read that recent thread of this month and you'll learn a lot. If it is gaining RPM's when you have it throttled down, then that's a whole other issue. Could be the linkage, governor, etc. But first thing first. If its fuel flow, you'll learn from previous thread about possible solutions. Mine ended up being a blocked screen at the bottom of the fuel pick up line in the tank. Easy fix. Good luck.