treiberg,

have you checked your oxygen sensor? Since your car is low mileage and 18 years old, I'm sure the O2 sensor is original, since they only last about 100,000 miles now might be a good time to change it anyway, even if thats not the problem. My 87 300e had a shot O2 sensor when I first got it. It ran rough and got poor mpgs. New sensor, problem solved! Otherwise, you can try running a few bottles of Seafoam through the gas tank, It worked REALLY well in my car (smooth performance, instant starts). Also, I hope you're running 93 octane gas...
Anyways, other things to check are vacuum leaks (which you've already done), EHA valve on the fuel distributor can crack and leak with age (regardless of mileage), and make sure the airflow meter plate is clean, and of course the air filter. Also, do pull the plugs and take a look just in case, because I have seen them fail.
To see if you actually have a completely dead cylinder, you can test each one by removing a plug wire one at a time with the engine running. When you get one that makes no difference, that's your dead cylinder.

Hope this helps