old thread, but i'd thought i'd add my woes.
i have 2 x thermaltake 3.0 riser cables and i also had many problems with my 3090 and msi x570 board (on a thermaltake p3 openair case).
the first cable that came with the case caused crashes, black screens and was not stable from day one. i ordered another thinking it was just a doa cable (after dumping 2000 gbp on a computer, i thought lets throw more money at the situation).
anyway, the second cable worked ok and ive been using it for a 2 years.... however recently i was looking to optimise my Apex Legends FPS and found out via GPUZ i have been running at PCi x2 speed! tried cleaning all pins with isopropyl alchohol and female sockets with air guns, but still x2 even after forcing the PCI gen 3 on the bios.
have had to dump the vertical riser on the p3 and just sideways mount (3 hours of #### work as i had to drain the water, re-cable, cut more pipes to support new angle, remove riser shelfs etc etc). now the rtx 3090 is in the pci port at x16 4.0 pcie speed.
#### things, i wont use thermal take risers any longer..