A relative of mine has an HP printer that seems to have an intermittent connection problem.
Every now and then it just loses it's connection - as though the cable has been unplugged. We've tried it on a different computer and that seems ok, so the problem is more than likely with the PC. We've changed USB cables and that didn't help.
The PC we tested it on (successfully) was running Vista, the problematic one is running XP. This leads to me to think that it's either a driver issue (seems unlikely?) or a problem with the USB card in the PC.