It could be that you are going below the low voltage cutoff setting of the speed control. If that is an adjustable setting on your Hobbywing speed control, you should confirm where it is set. Since it is happening 3 minutes into the race, it could be related to battery voltage (or even a bad battery).
Here is a link to the Hobbywing website (http://www.hobbywing.com/index.asp
). You might also check to see if there is a firmware update that may address any issues with your model of speed control.