When you ‘lose the connection’ just press button 1 on OpenSprinkler to see what the IP is. If the IP is still there then it could be something to do with your router. IP ARP entries on any router do timeout after a certain period but it should relearn it after the first packet is sent to the destination IP. The same goes for the MAC address.
If the IP is missing from OS then the problem is on that side.
Ingo