Got a "check engine" light that comes on/off... but mostly "on".
How can I determine what is causing this?

Oops... forgot to say that my car is a '99 E300 with 45,000 miles. I've changed oil/filter every 10,000. And, recently changed air and diesel fuel filter.

take it to a mechanic that can read the code. You may be able to find an autozone or napa that can do it for no charge. I get my codes read from a local MB mechanic who charges me 1/2 hour labor. Usually the code is for something I can do myself and I'll end up saving hundreds of dollars versus the dealer. Chances are it's a mass air sensor or something easy. Once you get the codes read, search this site for the codes to understand the problem and find a fix. members on this site are great at knowing what's an easy fix and what to leave to the pros... But the first step is to get the codes read.

AutoZone has OBDII meters for reading CEL codes that you may borrow to read codes to assist in determining CEL signals. Very easy to use and helpful. A very busy service saving consumer many dollars.