RickJamesBTC, did you ever try running windows to see if that works. Just curious because I may be in the same situation as you except I'm running windows 8.1 with 8GB of Ram. I just bought 2 of your usb risers to test and they get delivered today. At least I know your risers worked for you.
Someone told me he had trouble with some long ribbon risers and 290s. My 8x-16x risers work, they are ~17cm and have 2 capacitors but my 1x-16x risers are 30cm and have only 1 capacitor. I thought the extra capacitor might be reducing the extra 'noise' the 290s introduce (speculation) or the length could be the problem? Although all the risers work just fine on a test motherboard. Very weird. I had to order the long USB risers from you because that was all that was left in stock, has anyone reported trouble with the risers due to length?
Long shot, but do you run a Corsair AX1200 or an Antec HCG 900? I noticed in your thread about the risers someone was having trouble with that Corsair.
Gazza1, is this the modified driver you are talking about?
https://litecointalk.org/index.php?topic=11761.0