Hmm, so it could either be a hardware or software issue? I'm currently running a slightly more bleeding-edge configuration (KP51r1 + CSSU-Thumb), though the same issues occurred back when I was using KP50. I haven't tried to connect the N900 with stock kernel before, though I can't imagine any reasons why KP would introduce USB issues.

If it is a hardware problem, is there any way of safely taking the USB port off and resoldering before it breaks off and takes the solder pads along with them?