how to find out which 1 is gone

[zoaman]

I have a 50a esc running on 4s with a 650kv outrunner
this is what I have from specs and my findings.but some thing don't add up
16.8v battery fully charged
0.053 ohms on the 3 wires (correct as per spec)
1.8a per wire at 10.8volts from esc (correct as per spec)no load
now on no load motor should rpm at 650x10.8 =7020
I have 10000 no load
then when I add prop I only get 4750rpm
I have reset esc to factory, battery reduces equally per cell
the problem I have is, it seems as though I only got 2/3 power.
this happened in mid flight from fully working to 2/3 power in a instant
just like a low volt lipo.but was still well above low volts
any help is apreciated