I've had a similar problem before, but switching USB cables didn't work for me... Another option to keep in mind is to come to ppcgeeks directly on your phone, download the ROM you want to your internal storage, then flash using internal storage. It's a handy technique to know. Also, be sure to select "contacts" when using activesync
Also, maybe check out another backup utility. I mostly rely on activesync, but if you need to reflash while away from your computer, it's no use as of now. I use PIM backup, but everyone has their personal favorite.
To flash from internal storage:
http://forum.ppcgeeks.com/showthread.php?t=40490