I also have an update on my missfire of our 98: I cleaned the MAF as indicted above and the car runs fine up to a 1/4 tank. Below 1/4 tank the misfire starts again. I start to have the feeling that there is something in the tank (water?) that kicks into affect when it is getting 1/4 or below. My next step is to completely drain the tank and see if it helps.

 

I would really advise against draining the tank unless you can do it without modifying the tank in any way. I don't believe there should be any type of fluid in the tank that would cause a misfire only when it reaches 1/4 of a tank. If you have been refilling it, the suspect fluid would have eventually faded out with the other 3/4 of a tank that has been used.

That being said, it may be necessary to get a new mass air flow sensor. I read about a guy whose car would misfire if he "pressed the peddle too fast" (the same problem I am having btw). He's done a lot of work on his slk and it helped to clean the MAF but the problem wasn't solved until he bought a new MAF. Then his problem was solved. I'm going to try that myself soon.