I was running one card for months without an issue. Sometimes it happens when not mining at all.
Pretty detailed report. The quoted text seems like the key point. It suggests the problem was triggerred by the
installation of the 2 new GPUs. It could mean it's the number of GPUs or a specific GPU. Try removing
one or the other new GPUs to try to stabilize the system. You could also try swapping risers and slots.
There should be a pattern that leads you to the cause.