Be aware that many things can look like an ESC/motor stopping.
On PWM, I had one signal wire that was intermittent, and when it went open, the quad would fall, but always with a random motor seeming to cut out first (slight tumble).
Enabling Low Voltage Cutoff can cause it. Due to manufacturing tolerances, one ESC will cut off before the others.
High throttle on old / small battery can cause voltage drop and any of ESC / FC / Rx can reboot.
On either of the previous two, you will notice it much worse if you try to take off without recharging the battery after an incident. Tell us if this is true.
Props too large.
Do the motors get hot? Anything that causes the motors to get too hot from just hovering is an issue that can cause this. Props too large is one such issue. After just hovering for several minutes you should be able to hold the motor firmly without getting burned.
ESC firmware doesn't like that brand of motor. Sometimes you can hear a sync failure, sometimes called cogging. It sounds a bit like a stick being thrown into the spinning prop. Let us know if you hear that.
Excess vibration at certain throttle but not others (and on banking) can cause bad sensor readings.
Loose motors or damaged arms can have similar effects, but not crashing generally.
Loose props.
Metal parts shorting, like ESC connectors together or FC board touching metal or FC board mounted with metal standoffs.
RC radio glitches, but this is not common on 2.4GHz which generally is digital and has checksums.
Logging can help to tell you what was the cause if the FC caused it, or what was the first thing the FC detected (e.g.bank forward and left if that motor fails), but for you that means buying a pair of OpLinks.
I suggest you make a video of it happening and post it here.
If it's easy to re-create, you could firmly attach the quad to a table, etc. and run it in Rate mode (not Attitude) and recreate it on the table, then try some things, like Output page test mode that bypasses RC Rx and FC sensors, or Manual flight mode (bypass FC sensors and stabilization only) (which you should never do otherwise, uncontrollable for a human without a lot of setup). Manual might be allowed if you do System->Settings->FlightModeSettings->DisableSanityChecks and press green up arrow at top of window.