|Your terminology might be a bit wrong here. You say it is an Ad Hoc network and then go onto say the the computers are connected via a router and a switch. |
That is an infrastructure network.
An Ad Hoc network is were computers are connected directly to each other with no routers or switches involved.
Intermittent drop out in a wired network can often be traced to a faulty cables so that is where I would start looking, especially as it always one of the computers connected to a particular switch. I would look at the cable connecting that switch to the router.
Upgrading to Win & pro would make little difference, you would still have the peer to peer network that you have now and would do nothing for intermittent drop-outs.
You can not solve a hardware problem with software.