i think if i was trying to figure out an isolated problem like this i would buy/borrow/rent the very ones that are going through 3 and 4 mmm's. hook up every test probe you have and go bashing.
my 808 and xt8 v2 and v1 on 5s have never even givin a hick up and i use them at least 3 or 4 times a week. mike test drove them also.( chasing cars on the highway at 50 mph
) fan on fan off they work and work smooth.