Hi andya, i am experiencing the exact same problem with my 91 reg 851, nearly had me off twice last week on a run out, have tried to monitor when it happens the most, cold/hot/wet/dry weather conditions or when the bike is cold or hot etc, seems very intermittent and thats the thing that catches you out. Went all the way to Assen and back without a glitch and then happened twice on a local run? the only thing that i can locate is it seems only happen on wide throttle openings in 4/5/6th gears if anything. My bike does backfire at times but i thought it was probally due to it running race pipes/air box mods etc...
Going to monitor this thread with interest..