RZR 800 Missing bad 2500-6000 RPM’s

My 2011 800 RZR falls on its face when going Wide Open Throttle (WOT) at about 2500rpm to redline (about 6Krpm). It stumbles and misses throughout the entire rpm range, seems to get worse when hot. (but still bad cold). Changed the plugs, the missing and stumbling is less pronounced but certainly still there throughout the RPM range. Any advise would be appreciated..