KDX220 top end question

I own a 99 KDX220 that i bought last november and am finally starting to ride, i had previously owned a 91 KDX200, anyways i've noticed that when i pin the throttle on my 220 and it revs out it doesn't seem to like it, it's like it hits the top and almost bounces, like on a car when you rev it out it hits the rev limiter and kind of bounces a bit, it won't stay at the rev limit it kind of drops down a little then hits the rev limit again, that's what the bike seems to do when it hits the rev limit, bike has airbox cover modified, boysen reeds, rb carb mod, fmf rev pipe, and fmf silencer, i have not taken the top end apart to exmanine it yet becuase otherwise the bike runs fine, and the guy i bought it from said he had the top end done last season. i plan on taking the top end apart soon though but i dont' know if i'll find anything, so any input would be great

i do have the gnarly rev pipe and the guy i bought it from had the wiseco pistion put in, it's got fresh gas too, 93 octane, so i don't know, my only thought was the kips but that shouldn't effect top end i would think, could be wrong guess i'll see how dirty it is when i take the top end apart