My mom owns a 1994 C280. Of the many problems it has had, these remain most noteworhty to me-
at about 95K-100K miles it would randomly shut off when you would make a sharp right hand turn, as you may when manuvering into a parking space. This occured about three times.
At 78k the car had suffered from hard starting- it took the starter motor considerable amounts of time to get the engine to turn over, and once started powerful fraigrances of gasoline and paint thinner like odors emit from the car. It had been taken to the dealer, and it started properly only one time after. The problem slowly dissappeared after a while.
At 121K miles the car became non-drivable- The "check-engine" warning light lit up, and the car began to behave oddly. It would stall, shake, shudder, emit smoke, randomly accelerate and slow down. Most of the time if the car wasnt running at high RPM'S or kept at or above 25MPH it would stall. The dealer believed that it required a new computer- a big $$repair$$. So the car retired fromn service and resided in the garage, while my mom used the bus to commute or my 1984 w123 300D to run errands until she had enough money to have the car repaired. Today it comes home from the shop- hopefully it will be the end of its problems.
This car has been properly maintainded, and has continued to reveal new problems. I dont think I or my mother will ever buy a car built in its first couple model years, no matter how noteworthy the manufacturer is.
-I am interested to know if these problems are familiar to other C-class owners?
-Nate

The problem is that you have been taking the car to the dealer. You need to take it to a trustworthy independent mechanic.
I can tell you right now that the frequent stalling and not starting is more than likely related to the OVERVOLT PROTECTION RELAY, which is only $50 and you can install it yourself.
The check engine light could be a number of small things, maybe the O2 sensor? WHich is about $150, I jsut a new one put in mine. You will need to know for sure what the codes are by using a hand held OBDII hookup, which the mechanic will more than likely possess. Your check engine light could also have come on from a rough idle, which could be caused by an old fuel filter, a $20 part commonly looked over.
Have your mom take it to an independent mechanic. Poor lady! It will hopefully be what I have listed above, a somewhat small amount of money, and not that big of a deal. Just don't take it to the dealer unless absolutely no other place can resolve the problem. GOOD LUCK!

I own a 1993 C class (diesel) and although I didn't have any of these problems, I can second the thought displayed above...
The 1994 US-C class was already a lot better as the (very cheap equipped) 1993 european-C class.

The only advice I can give you is the same advice as given above: start exchanging/renewing the dealer...

greetingz,