it could possibly be running rich, maybe too much fuel after 6500 rpms fouls the plugs?

i'd have a look at your carb first...

but on another note is it possible to swap around the spark leads? do that first if it is, saves you from buying one

but heres a link to my old post when i was fouling plugs

http://nsr250.net/forums/viewtopic.php?p=46772&highlight=carb#46772

hope it helps