I'll have a go then.
If it is happening on both fuels then it could be an air leak causing both fuels to go lean. have a good inspect of all the hoses etc related to the plenum, breather from rocker cover, stepper motor hoses.
Or
It could be ignition related. The prime suspect here would be the ignition amplifier or the coil. Is there a time element to this as in it only shows after the engine has really warmed up.
Steve