I had a similar issue before, did all the "known" solutions (new drivers, check risers, check power, check overclocking profiles and even tried stock); my rig would always crash after installing the 6th card.

Anyways, It seemed to work fine after changing my PCIs to GEN 2 instead of GEN 1, and disabling CSM Support from the boot settings in bios.

Whats still bothering me is that I dont understand how this solved my issue, it just does not make sense; but it worked on multiple rigs.

In your case it could be worth a try.

Good luck