It could be the battery or the ESCs low voltage cutoff.... One other thought (!) your Transmitter has fresh batteries?

The XRB packs are kinda odd in that they have 3 wires. I THINK the Hirobo charger may be charging each cell individually - so it needs 3 wires.

If the pack is defective (manufacturing fault or over-discharged) then it will start off showing 8.4v but will quickly 'sag' as you draw current thru it... Once it gets down to ~3v per cell (6v total UNDER LOAD) the ESC is going to trigger the low voltage cutoff. The problem is that you will need to test the voltage straight away (within seconds of the cutoff) to verify this as the pack will probably recover quite fast to 7.x volts. The only way to do this is to have it strapped to a desk and to 'interject' some voltmeter probes in series in the connection from battery to heli.. I've done this with my own pack connectors in the past but the Hirobo connector is 'different'.

A defective ESC is harder to prove. I'd suggest trying a different pack ... if a second (known to be good) pack also sees these symptoms then the electronics are suspect.

If memory serves the XRB uses 720 MaH 2S packs.