Quoted from boagman:
If you follow the Service Bulletin on Stern's website regarding the drop target assemblies modification, there's no more tweaking necessary. Yes, you need to do all of them, but once it's done, it's done.
Yes, I shoved some very thin washers behind the 3 or so banks than needed it (3-4 places) and it solved the problem for me forever (*so far*).
Once I realized I could just loosen a few screws on the black plstic bracket, slip in the washer, but a screw through it... it took very little time to do.