Dell Optiplex GX280 Mouse freezing

Hi. We have two of these GX280’s. The one is working fine and the other, not so fine. We just replaced the harddrive on the one and re-installed the system. This PC is used for GIS mainly. We find that when the machine is under load, the USB mouse freezes and you have to unplug and re-plug it. Very unproductive. Does anyone know what is causing the problem?
Thanks.