bogging problem at high revs (beast)

[the_ripper]

so i still havn't fixed the problem i origionally joined this site to find a fix for. my beast runs great but when i pin it out it seems to rev high then bog out randomly it will be good for a bit and then bog out again. any ideas for a source or cheap fix? i have changed the spark and that helped for a couple of rides. should i try an oil change seeing as it has been sitting throughout the winter?