Pin 13 flashes some number (never counted them) on reset but the blink sequences is like blip,blip,blip,blip,blip. in about 1 second.
Maybe it's reseting because the power is dropping. In case you haven't figured it out, the purpose of the jumper is if your motors need 12V and arduino dc ext pwr jack is best run on 7 to 9V, you can remove the jumper, run the arduino on 9v walwart or battery pack and run the motors on 12V.