Sigma 50mm f1.4 Focus Issues - Any good copies?

I purchased a new Sigma 50mm f1.4 (Canon mount) from a camera shop in town (so I could return it if necessary), knowing that there can be focus issues with some copies.

Indeed, there were issues. Everything was fine up close, but pretty much every time I focused on something beyond 15-20 feet, it seemed to just go to the back of the room (infinite?). I could not get it to autofocus properly at those lengths and beyond. I tested it on my own XTi and 40D. So, I took it back to the camera shop, and we tested it on my bodies and their own 7D - same issues, and they quickly detected the issue as well (so I knew I wasn't crazy).

Long story short, they sent it back to Sigma, and I waited a week for another copy to come in. The next copy came in, and now that I knew what to look for, I tested it at the camera store (again with multiple bodies), and we had the same exact issues - great focus up close, but completely inaccurate/back-of-the-room focusing past 15-20 feet. I was about to try sending that one back, and try a third, but one of the guys at the store was like "Honestly, I can't tell you that we're going to get one in that won't do that."

So, my question is, is that true? Or, does anyone have a copy that does indeed accurately focus at all lengths? I have the Canon 50mm f1.4, and I found the Sigma to be far superior in sharpness at the high apertures and to have much better bokeh, so I'd love to use one.

Thanks!