Problem first started yesterday after about 20 minutes of driving. I slowed to a stop light and noticed the engine was running very poorly... seemed like it was misfiring badly, shaking the whole car. I pulled over, turned off the car, restarted it, and it ran fine. Today, this has occurred about 5 times. Once starting the car, it runs fine... but after a short time driving, and coming to a stop, I notice the it not running properly. It is not only at idle... the engine runs poorly at all speeds, there is quite a power loss, and transmission shifting has harshened dramatically. No check engine light has come on or anything of the sort. I don't think it's the spark plugs, fuel filter, air filter, or anything like that... because it runs fine at first... and then suddenly begins chugging.
Any ideas? I hope it's nothing major. BTW, I have 110k miles on the car.
1995 C280.

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
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!)