Only happens occasionally at the moment. On gentle acceleration when still cold (about 10 mins running) miss-fires at about 2500 rpm. Boost gauge =10psi vibrates like mad. Back off acelerator and it stops. No check engine light. Goes away completely when warm. MY00 49000 miles, plugs changed at 23000 (standard). Only mod is open kneck downpipe. Anyone help?

Thanks, Graham.