I think it may be a spark problem. Try replacing the spark p'z, with Bosch Platinum 2's, and since they come spaced, only take a few minutes to change. Platinum 4's in my opinion are a waste of money...if you have to reley on 4 prongs to makes sure your car gets proper spark, I think you've neglected your car to a point where a mercedes tech should fly out to smack you upside the head....but this is a toyota, you'll live.