I’m running Nuendo 6.5 on a Hackintosh, Mac OS X 10.9.5, UA Apollo Quad (thunderbolt). Sometimes I get momentary dropouts. When that happens, I shutdown the computer and power cycle both it and the Apollo. Problem usually goes away for a few days, then returns. I’ve always chalked this up to a problem with the Hackintosh/thunderbolt connection, but I’m not sure.