cut off is 4.2 per cell....i think. i will double check when i get a min. later. it has 2 motors and both are brand new because the unit is new. i dont have time to type now, but i will respond with more in about an hour. thanks.

Hmm, cutoff should be lower, like 3.6 approximately, certainly lower than the full charge voltage.
Nothing is bindig is it, like are the gears too tight?
Even new motors can be bad, not likely, but can.
Actually, if the esc is cutting off at 4.2 there must be something wrong with the esc, like too high a cutoff point.

no, nothing is binding. When a full charge is given, (at this point just 10 min.) the xrb works perfect, but then shuts down after about 2-3 min. Like i had said before, everything was working perfectly (esc was cutting off after about a 10-15 min runtime, but now it cuts it after about 2 min. Then it takes about 10 or so min to charge the battery back to full. Readings are 8.4V. 4.2 per cell. When the problem started it went from working perfect to doing this in a matter of a second. one second it was fine, the next it was broke. It wasn't an intermident problem.

Sure sounds like the esc is at fault.

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.