I agree that first test should be testing the battery...since in inductive circuits (motors) when voltage drops, amperage goes up. Test under heavy load to see how far the battery drops. If the battery is not the issue, you need to take a hard look at the flyback diode DF1, its associated snubber SN1 for shorts or leak through (a Handyman works great for this). Also it could be the field windings in the motor shorting out. Keep in mind...what kills transistors and other semiconductor components? Heat via high current flow...the load in the drive circuit is the motor, therefore if motor is shorting, you will be pulling high current through the transistors. Same thing for a shorting DF1 flyback diode, it would be bypassing the motor which is the load. Good luck.