just changed downpipes on my 600fx. bike has been stood a while.when i ride the bike 1st gear is fine so is second but third gear if i hold 6000 rpm for a while then accelerate hard the bike shakes and hessitates but if i gear down and get the revs above to 7000+ its fine but does the same in other gears too.have changed the plugs but makes no difference.smooth on idle too

any suggestions??

 

got the same problem on my cbr 600fx after changing my down pipes as the old ones were rotten so any help would be gratefuuly appreciated.