It seems like you monitored CPU/motherboard temperatures, not GPU, which is the cause, as you get graphical artifacts(the rainbow things).
Get "MSI afterburner", it has temperature monitoring for most GPUs.
You can also use their bundled "msi on-screen display server" which shows gpu and memory usages, temperatures and few other things while in game, and monitor that closely when you start getting those artifacts. If the temperature doesn't go over 90C before you get a reboot or artifact, it is most likely either insufficient PSU power, or faulty GPU. In that case, try to borrow other PSU's or at least underclock the GPU and check again.