Fazer lad

Don't all rush at once now!!!!

I have a 1999 Fazer 600. It has 34,000 mls on it with full service history. While out today when accerlerating hard at about 8500/9000 revs in 4th,5th and 6th gear there was a slight power loss. If i ease off and slowly roll on the throttle it seems to be okay but while on a stretch of dual carriageway i could only just manage to get it above 9500 revs. Has anyone encountered this problem????? as it is quite annoying!!!!