If you have firewire ports on both PCs then you could buy a cable & setup a private firewire link between the 2. Instant fast reliable 400Mbits/sec private LAN.
BTW a network being a little slow should not cause MP to disconnect. It might make the mouse laggy when the network gets slow, but thats unavoidable. Only a network which either loses packets, or takes more than 5 seconds to do a 'ping' will cause a disconnect.
If you have quick detection of disconnected machines on then the time above switches to 100ms or so. So don't enable that on a laggy network!
We might add a switch to prevent MP from ever discovering a disconnect and see if that helps or not.