I think mine has a 54mm system & ecu too (I say 'think' as the box the standard pipes are in says 50mm on it, but the previous owner said it was a 54mm he asked & paid for). Also has the air restrictors removed.
It doesn't do it all the time, and I did wonder if I picked up a dodgy tank of fuel. It has done it on 3 seperate occasions now though. I also think it is running rich as last week in the hot weather on overrun or very slight throttle off it would pop & bang like a goodun (the french villagers loved me !
).
Hopefully a new set of plugs and the pc will fix it, but just thought I'd ask.
Cheers,
John.