Have you tried something similar to my design? Yes Any problems with cooling? No And if the GPU is facing down, how is the cooling? Great Is there enough room under the video card and the bottom of the case? Yes I'm guessing all your video cards were using pci express 16x, right? Yes Different cables helped or it didn't matter? Didn't matter A problem of length maybe? Not sure Or not enough shielding on the ribbon cable, a problem of quality? Yes After all, if the normal riser cards work fine (as i assume from your design), then it's something to do with the ribbon cable used in flexible riser cards, but what? Lack of ground plane My PCIe extension was much shorter that what you intend to use. The first sample I recieved didn't have any shielding, much like an IDE cable. Without any shielding, it wouldn't work at all and if the board managed to post, it revert back to the onboard video as if the gfx card wasn't even installed. I tried placing aluminum tape to shield both sides of the ribbon and although this helped significantly (bios would find it every time) there were still problems with stability and some cards still didn't work at all. Symptoms would be the computer hanging, bsod, video goes black, reboots, or powers on with no video. It has to do with signal losses, echoing, and cross-talk. Some cards can apperently handle a little noise, some don't. I've seen someone use an extension that was 12" long and it worked fine for their setup but this was back in the GeForce 5k/6k series days. The riser cards I've got my hands on all have a ground plane layer in the pcb and some even have additional capacitors on the power rails to provide a little extra filtering. Lets say a card's ground reference floated above zero, say 0.5v, and the logic signaling is supposed to be between 0 and 1v, what you wind up with is signal switching between .5 and 1v. This is one possibility that would reek havoc on a digital circuit but I'm not a digital signal expert. Interestingly, Thermaltake uses a ribbon in their Mozart SX (VC7001SNS) chassis. The toughest thing about designing a product is the trial and error and the learning curve. If you have engineering experience it's alot easier but there's still hurdles to overcome. If this chassis was a standard ATX chassis it would be a cakewalk but it's not. I've had to nudge things .005" here and .020" there and really know beyond a shadow of a doubt that everything is going to function as designed before I drop a huge investment on parts. The devil IS in the details. I've seen him!