is this happening while you are imaging your machines? ie: heavy internal network transfers?
Try power cycling both devices. Power off the modem. Power off the gateway. turn on the modem. Wait for blocksync, then power on the gateway. The problem with modems is that they will register the first device plugged into them, ie: your previous router. If your modem has a reset button on the back of it, you may want to try that afterwards.
As well, you may want to upgrade to f/w 1.2b05, stay away from 1.3 unless you are fine with the consequences. consequence of not downgrading back to previous firmware if something doesn't work out for you.
if your still having issues, try setting the dns server to an OpenDNS server.