I would not be surprised if it is the +2s causing the problems. If you search this forum you will find all sorts of stories of people using +2s and +4s and having misfires, then when they switch back to the OEM style plug form Beru or Bosch, the problem disappears. If the problem was not around before the plug change, I think we found the issue.

The OEM plugs are expensive (around $9 a piece) but worth it in terms of lack of headaches!