i really don't believe it's a temp thing, mid 60c is perfectly fine, im running 60-65c across all my cards.
now when you say the system dies? what happens? stutters? freezes? cant use it at all and a forced restart is needed?
The rig in the small room freezes. can't move the mouse sometimes. screen goes black and just have to power it down and force restart.
a couple cards run at 78 degrees, couple at 75 degrees and some near 72 degrees. depending on where i put big fan.
I've reinstalled drivers a few times.
i have the same board, fantastic board BTW with the latest drivers ofcourse. Those cards i believe are running hot, i dont like to see anything over 75c, damage doesnt start to kick in until your nearing 90c.
the slider alone is weird as heck! you should always have control of that thing no matter what, i kinda wanna say software issue but can also be hardware that isnt working properly.
You can start by lowering to the settings i sent you, then if that doesnt fix it, start removing card by card, then swapped risers. process of elimination.