Have you checked that the DHCP server lease duration to provide IP address's to DHCP client machines is set correctly. Because if the lease duration was wrong then client machines might loose their IP's and possible the connection too?? I'm NOT saying this is definately the correct diagnosis here but it is something that you should definately check on.
**Check your DHCP server's settings for how long the lease duration is?
**To check the lease duration on a client machine goto a DOS window or open CMD and at the prompt type in the following command IPCONFIG /ALL the press enter this should give all the TCP/IP information including the lease duration.
**Secondly you didnt say what practical function the dongle on the PC machine serves? i.e. is it essential and if so what if any bearing might it have on dropped connections if any?