Chip,

could be alot of things but sounds like a short or that the throttle
body portion near the top that houses all the ASR and cruise is faulty.
When it senses slippage, or fast turning with lots of acceleration it
bogs the motor to limp mode, what it sounds like its doing.  It is to
protect the driver.  First replace all the fuses, who knows that
normally cures alot of things.  Then if its not that easy check your
wiring harness for shorts in the engine compartment.  If its not that
try that intake/MAS/cruise ASR throttle body.  If its not that maybe
the ECU.  If you get frustrated take it in, Mercedes or advance auto
can hook it up to a computer and hopefully pull a code and tell you for
sure.

Jason

This is an electrical problem. There's a relay called the "over voltage
protection relay" (it has a fuse on its top) that causes similar
faltering when the OVP relay fails. I suggest you do am Internet search
on that topic and see if the car's symptoms agree with other owner's
experiences. This car is of the right age for this and another relay to
fail.

For future reference the other relay to keep in mind is the fuel pump
relay. It becomes intermittent - much like the current problem - except
there's suddenly no fuel and the motor stops - or doesn't start at this
try but will start at the next try etc.

The fuel use is bogus to the problem. The economy gauge is a simple
vacuum gauge. When the motor bogs down you instinctively step on the
accelerator which opens the throttle plate and so reduces the engine's
manifold vacuum. That's all it is showing you.