If you're looking for advise, I would advise taking it to a shop, if it really bothers you.
There were/are so many problems with the 98 ML's, there really isn't one or two common things that would cause this. Or I could tell you to check.
You ask "what could it be?" - any number of things.

Is the problem consistent? Does it miss more or less while accel/decel/ idling??
Try to get some more info and really narrow it down.