I now have two bike with horizontal dropouts and both choose to slip a bit causing the rear wheel to cock to one side.
One of the frames is a Battaglin and the other a Contini. (spelling could be a bit off)
Have I tried to tighten the system down, Yes. I have in fact really put a lot of pressure on the hubs and am using steel skewers. Both frames are set for 132mm and the hubs are slipping in what seems to be fine. But when I am done with riding, I find the right side of the hub has pulled forward sending the wheel off center to the left by the power of the crank pull.