[Z-linkCobra]

Its generaly spark or fuel related. Have you checked the valve lash? Might want to pull the valve covers and just give them a once over to eliminate that possibility.

If the lash was too tight then that might cause a valve to hang open a little and give you grief with reflux at the lower rpms. If it was to loose then it can retard the valve timing and give you a backfire through the exhaust...Im just thinking out loud really. Just something to look at.