USB Ports

My customer is running a program that requires a com port (Serial Port). we
are using a USB to serial adapter configured for com 4. We are experiencing a
problem that disconnects the external device after about 4-5 hours forcing us
to do a system restart and a reconfigure of the external device not the USB
to serial adapter. we have disabled the hibernate feature and can only think
that the OS is disabling the USB ports or something to that extent (not
sure). If anyone would have some feedback about the peripherals and there
operations on Vista it would truly be appreciated.

Thanks

Mike