Just a quick update. I received the my board back from QC and it was indeed a bad BL ctl. New one is on now and working great. Thank you everyone for your help.
Hey Darren, I think I'm experience the exact issue you have just gone through. Perfect few flights. Land, change battery, start up is normal ... except for one motor that seems to be stuttering. I switch motor wires with another and it runs fine, except for the switched motor which does the stutter thing. All BL lights green. MKTools shows no BL Errors. I've got some spare BL's so I'll swop it out. Did QC give you any feedback as to the cause of the fail on your BL? Or is it just one of those random floating point errors that can't be explained? Cheers.