Alex, here is what you need to do.  Tell us what year your car is.

But assuming it's pre '92 from you describing it as a 300E:

 take cover off X11 on your left front wheel well.  put a scope on pin#3
and pin#2 and look at the duty cycle.  Put an appropriate (forgot the exact
size offhand) allen key down the open hole in your air filter housing and
turn the screw 'slightly' clockwise (I think that's the right direction, but
you will see the changes in duty cycle on your scope) you probably should
not go below about 40% or thereabouts.
 You could use a voltmeter, but it might be difficult since the voltage
might jump around a fair bit making it difficult to estimate what your
duty cycle is... on the scope you can see this nicely even though it jumps
around since you are simply looking at a 10 ms (100Hz) square wave
 
If this is all gobbilty gook to you.... take it to your dealer.

cheers, guenter

Change the oil and all filters.

Add some gas line antifreeze and use premium. Take the car to be
tested HOT after it's used up the tank of fuel with the gas line
anti freeze.

New plugs wouldn't hurt either.