I have seen posts in other threads, where several people resolved similar issues, on Windows at least, by plugging into the main USB ports on the back, instead of the expansion ports on the front or the lower-back. A few others had problems from conflicting devices (external USB HDD, for example). Their problems seemed to stem from low voltages or other conflicts.
Based on these posts and this link:
GigaMegaBlog Simple fix to iPhone/iPod dropped USB connections
I ordered this externally powered USB hub:
Amazon.com: Apc 7 Port Hi-Speed Powered Usb 2.0 Hub: Electronics
If it makes a difference, I'll let you know. If anybody finds a better one, which solves their problem, please let me know. ... I also ordered another mini-USB cable to see if that makes a difference.