You say you "surmise" that you get these speeds before and after and directly connected. How are you acquiring these speeds? Are you using speedtest.net to gather your information? I would use this test if you are not while directly connected to the ISP modem and while you have the 685 connected and do some testing here and there.
I presume there could be an issue with this ISP modem/router and while using the 685.
One thing you can do is if you have a friend or nieghbor that you know with broadband internet, take the router to there place and set it up. Hopefully they don't have the same ISP modem would be preferred. However this would be a good test to see if the issue tracks with the router or not.
You need to see if you can find out why the router is slowing down after a while. I don't think it's the router as it doe connect and does maintain the speed for a period of time.
Another thing that you can try since your ISP modem seems to have a router built in. See if it has a DMZ option. Put the 685 routers IP address that it gets from the ISP modem in the DMZ and see how it works. You'll need to unbridge the ISP modem if you haven't done this already. This will let the 685 have everything from the Internet unfiltered.
And you don't have to wait for GB internet to have CAT6 cabling. CAT6 is fully backwards compatible with other speeds and just having it gives you some overhead on data connections and speeds too. I upgraded my LAN side couple of years back for under $40 bucks here before I got my Motorola SB6120 Gb modem.
Keep us posted.