See this thread for scanners that will work with our X.
http://www.xoutpost.com/bmw-sav-foru...i-scanner.html
Since it's a random misfire and not a specific cylinder that is misfiring I would rule out spark plugs, coil packs and injectors.
What causes random misfire? Lean condition such as low fuel pump pressure, vacuum leaks, clogged fuel filter and sometime the MAF if it is dirty or has water on it.
What have the stealers done for 15,000? Do you have the repairs bill indicating what's been done to try and cure this problem.
If you can list the repairs, we can see if it might be something else instead of chasing our tail.