That is a good point, BUT, his Check Engine light would illuminate if there was a problem. The O2 sensor is monitored by the CAN.
I think you posted before and I suggested you change the fuel filter, did you ever? What about plug wires (I'm sure you checked this, but..), and spark plugs of course? What about the air filter??
GOod luck and hopefully you'll find a cure!
2006 E350 w/ 155k miles (Daily Driver)
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)