Same issue here. GTX 660 (non-ti). Works only in black 1x slot, freezes after POST but before boot if installed in blue 16x slot. Everything ruled out w/my system (RAM good, PS good, etc.). Did NOT have any issues with old 460 (hoping to not have to go back to it). Both cards are ASUS cards, just like my motherboard (figured support can't ignore me if it's all ASUS). Both have nvidia chipsets (MB=780a). Asus M2N HT Deluxe + Asus DirectCU II GTX 660. Either these cards are crap or there's a BIOS issue. Biggest concern is that I will not be able to SLI.
Anyone find out anything? Sounds like it's not isolated to my particular motherboard.