Distortion? or Artifacting? When I think of distortion, I think of loose or bad cables causing blurriness/distortion. When the distortion happens, does the system totally lock-up? It's also very unlikely both cards are bad. Try removing one card, thoroughly test the card still in the machine, then swap it out with the other card and run some more stress tests.
EDIT: Also a bad SLI bridge can also cause distortion/image issues.
Gigabyte X570 Aorus Master | Ryzen 9 5900X | 32GB Crucial Ballisitx DDR4 3600MHz | MSI Radeon 6900 XT | Samsung 950 Pro 512GB | 2x Crucial MX500 1TB Raid-0 | 2x WD Velociraptors 500GB Raid-0 | Seasonic Focus Platinum 850W | Lian-Li PC-O11