You problem is using a usb buss. They work but inconsistenly. You need these adpaters linked here : http://www.newegg.com/Product/Product.aspx?Item=N82E16814998050 Your problem will most likely be solved. They do not rely on usb power and have been designed from the ground up to work with eyefinity whereas the adapters that require usb power were never designed or intended to be used with AMD cards or eyefinity. I have had 3 eyefinity systems and these adapters are the only ones that ever worked consistently. Cable length has nothing to do with the dropouts. The signal is digital and unless you using like a 100 foot cable or something you should be fine.
I have the Powercolor version of these and they drop out just the same. I think it is a driver issue sometimes on mine as they tend to de-sync a lot more on some drivers than others. It's annoying though when I'm watching a movie.