Originally Posted by tinypanzer

I don't know how old the motor you put in there is, but M103 motors get leaky valve stem seals after about 100K miles, and I have discovered that the oil leaking into the cylinders will foul out the plugs about every 3-4K miles in my 300SE. Taking them out and cleaning and gapping them gives me back power and economy.

These motors are very sensitive to the condition of the ignition systems so check that.

You certainly want to get your duty cycle sorted too. If you set it correctly a while back, it might be an indication that the 02 sensor is on its way out, or you've developed a vacuum leak.