USB power problem

[tkhoin]

Any help/comment would be appreciated:

PC:
===
- Brand: generic AM-Comps
- Age: 3 years old
- CPU: Pentium4 1.7Ghz
- MotherBoard: PCChips M935LR
- PS: MGE Vigor 400W
- USB: 2 original on the back, came with PC
- USB: 4 extra on the back, I installed a PCI adapter
- USB: 2 in front, I installed the extension bracket/
cable that plugged into a USB socket on the MotherBoard

Symptoms:
=========
- All 6 USB sockets on the back work fine with all
USB devices.

- The 2 front USB sockets seem to have power problem.
They work fine with USB card reader or USB flash cards.
However, when I connect a USB thump drive or a USB digital
camera, I can download (from USB device to PC) small files
or pictures (transfer time less than 10 seconds or so).
But if I try to download a large file or whole bunch
of pictures (transfer time more than 20 seconds), the
PC just shuts down and reboots. This would happen about
70% of the time.

It seems that these 2 front USB sockets can not handle
power demand jobs? What is the reason? What makes them
different than the back USB sockets?? How to fix this
problem?