Can you confirm whether you have set the router up with a static IP address supplied to you by your ISP ?
Or does it get its own IP address dynamically from the ISP ?
If the router keeps 'forgetting' the settings you have given it, either the firmware may need upgrading or the device may be faulted.
Do you think it's the router and its own IP address at fault, or is it perhaps that the router is tripping itself up trying to deliver wireless connectivity or when trying to issue IP addresses by DHCP ?
Are your machines connected to the router using static IP addresses ?
How to the machines authenticate to the router for wireless security and what range are they in (signal strength as opposed to distance) ?
Get back with more information and we'll see what we can do.