USB mass storage keeps disconnecting

Hello,

whenever I plug in my DOK or any mass storage through USB (including mp3) in windows xp it is assigned as a drive. however, after few minutes (while copying or deleting files) windows xp can't read it any more. browsing the folders fails and all other copy/delete operations fail.
sometimes I manage to prolong that period of time by a dos batch that runs in the background (dir *.* /s).

I already called a technician that told me after 2 days in the lab that this didn't happen to him however when it was brought back the problem occured again. he told that this is strange and that he doesn't think this is a hw problem.

OS = XP home edition SP3
DOK = Sandisk + U3
USB HD = WD 500GB
USB ports = 4 on board + 4 on ext. card
printer = HP
KBD + Mouse = PS2

Thank you.