What could cause the internet to fail at night?

For the past few days when it would get around 11PM the internet would start to fail. It would work for like 5mins then it would fail and keeps repeating resolving host. and i figured out if i restart the computer it would work again, but then fail again after afew mins.

Vista
dual core AMD 4000+
4gigs pc2 5200 ram

net.
high gain Gsky realtek RTL8187 wireless USB card. I use that to connect to my router that is about 300feet away. (I get full signal with my high gain card)

P.S. We been having a problem with our net for about a year but it has to do with our ISP (windstream) I think the modem is causing it. but it would drop the internet connection and pop up a window from the modem saying that it did not connect any DSL connectivity to the modem but then reconnect to the net in a min or so. and it does that all day. and gets worst with the more bandwidth pulled.

Also I don't know if this is tied in with the top problem but here lately my Wireless nic would disconnect from the router and i go to view wireless networks and the router would not show up but another one would show up (think the house next door) and it would connect to it fine and as soon as i disconnect from that router my router would show back up but as soon as i try to connect to it windows says my wireless card has a compatibility problem with my router when i never had a problem before.

 

DSL,eh? Call the phone company and have them test the line. Could be line troubles. Critters gnaw the lines, moisture (dew,rain) gets in the lines. The phone company should be able to see loss on the line.