Hi,
I have had the exact same problem as you have, and mine began when the new update occurred (THE NEW NXE BIG UPDATE). It was strange sometimes it would not work properly for like a week, and I thought, hmm...maybe it was xbox live. Then it worked fine for a while, then stopped working for a couple of days, then later would work fine. I honestly had no idea whatsoever what the problem was, I always thought it must be xbox live. So it got 2 a point where this was not on, so I had a search for some answers on the web and came across this post. I had realized only small episodes worked, like friends, lost, desperate housewives which I had downloaded, music worked like a charm too. So i thought maybe my router can't handle the large files properly, or maybe a bug with the connect360. So then finally a new competitor- Rivet. Purchased that, and unfortunately the same story....
...But what I had not realized was that I have my wireless adaptor attached via my HD DVD player which I still occasionally use for my HD DVDS and for the extra USB ports at the back. So I thought hmm...maybe the new update must not have had the HD DVD attachment properly fixed, since the HD DVD player is dead, I guess it is neglected and have some flaws in the update for that part. So removed my wireless adaptor and placed directly onto my xbox and it works like a charm. As a result, the connection must be poor via the HD DVD player due to some bugs/issues in the new NXE update because before it worked seamlessly everytime in the old dashboard without any issues whatsoever! So as for my newly purchased Rivet application for the mac, it works just as good as the long running Connect360...dare I say better? I think so!
I hope this helps for anyone who stumbles across this forum.