Loosing power

[Nic8]

I have a question, I am running 1/10th touring car 17.5 blinky and about 4 minutes into the race I loose power.
I can then be runing the rest of the race with full trottle but slower, so the question is, is it my esc, battery or motor.
The batteries have great punch to begin with and it happens to both, I did run a couple of meetings without balancing them, could it be they need a discharge and recharge to sort them out.

Cheers.