I replaced the single pickup roller supplied with my MTH RS-3's with an MTH double roller, in order to improve operation through switches. These and some of my other MTH engines would stall on my fastrack uncoupler magnet when operating at slow speeds. I would also get some stalling on trackwork that was a little above the normal rail height or when the trackwork was not perfect.
The cause of the engine stalling is that there is not enough space between the pickup roller and the plastic insulating piece supplied by MTH to allow the roller to retract all the way into the truck frame. The pickup roller would push against the truck frame and lift the drive wheels off the rail, thus stalling the engine. As a result, a slow moving engine may stop on the uncoupler magnet. (a faster moving engine would have enough momentum to overcome the friction at the magnet and pass over the magnet, even though the engine would "move or shake" a little bit)
The solution is to add a thinner insulating layer between the pickup roller and the truck frame. By replacing the thick insulating piece supplied with the engine, with a thin piece of plastic or electrical tape that is thinner than the factory plastic piece, allows the pickup roller to retract further into the truck housing. This prevents the truck from binding between the center roller and the raised uncoupler magnet.
I have include the fix in a PDF file for download.