I think my problem is solved. I reset my Roku...
by unplugging the Roku and from the wall, then I unplugged the router, then I unplugged my modem. I waited about 20 seconds, then plugged in the moden, the router, and then the Roku, in that order. I waited for the Roku to reset and then I tried all the staions that I had trouble with and much to my surprise everything worked.
I don't know how long this will last, and if I'll have to do it again, but for now things look great!
Thanks, and I hope this thread will help someone else