200 RPM drop then steady cruise

[Carguy89]

Basically, I'll be driving at a steady, say, 4800 rpm cruise. All of a sudden, it will drop to 4600, and be like cruise control, with it only varying if i take my hand off the throttle or if I really nail it or if I go over a big enough wave for it to completely come out of the water. It's done this since I bought it 2 years ago, and never thought anything of it, but it does seem to be doing it a bit more often. What is causing this?

 

[Bills86e]

RAVE valves need cleaned, check bellows. Should open around 4400 rpm. Bill