I'm building up an '05 Cannondale Prophet 4000 and I am having problem with the derailleur cable housing. I am using the standard 5mm cable housing and it doesn't stay in the cable guides. It doesn't fit and the guides don't seem to be the standard size. Does it have something to do with the fact that I'm not using a Lefty on it and it was originally set up for it? Or do I just need thicker housing? Thanks for your help.