After burning out the H-Bridge motor controller from Adafruit we tried the NYT version and had the same problem. We have started a new design based on the design by Radu Motisan at http://www.pocketmagic.net/
. We have laid it out in Eagle and built it on a breadboard and its working well so far. We are trying to add forward and reverse LEDs before we move it over to a Radio Shack 276-150 board. If you need the parts list available from Mouser let us know. Eventually this overheated when pushing dead weight at a full stop as well.