I suppose you could try swapping one of the misbehaving ESC's for one that is working fine and that will tell you if its the motor or the ESC thats problematic? Only other things I can think of is things like making sure the wires are in proper contact etc and that vibration isn't causing any problems with any of the connectors. If your 2 ESC's that are misbehaving have any shared connectors that the front ESC's aren't connected to I would be inclined to start there.