Could be thermal cut out on motor, or you are hitting the esc low voltage cut out, indicative of a tired/high resistance battery pack. Did you try more than one battery?
Check esc low voltage cut out and temp or motor temp cut out settings (if it has them). disable, (or lower cut off voktage) and see if it does the same in a race. besure not to run the pack down flat if disabled.
check motor and esc temp with a temp gun when it cuts out to see if its excessive.
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