I've had recurring problems with Roku not "finding" my wifi. PC, Ipad etc get it fine. After long discussions with no help from Roku or V tech support, I did a hard restart on the router and all was well. For about five months. Now I can connect the roku ONLY after a fresh hard router restart. This also required re-registering with Roku, netflix, etc.
I can fiddle with the router page. Nothing can be done to the roku, which suffers periodic updates.
Besides what has already been suggested, one other thing I would try is to try using WPA2 wireless security rather than the WEP.
I've had a couple of devices recently that would not stay connected using the WEP. But once I changed to WPA2 the connection remained stable.
Now dont ask me why. I have no idea why this has worked. The encryption should not cause it to constantly loose connection but the results speak for themselves. This solved the problem I was having with my Sony BluRay player and my Ipad. All other devices always remained connected but those two would constantly loose the connection no matter what I did, until I changed to the WPA2. Maybe it will help you too.