Xbox Live On-Going Problem?

My Xbox Live connection is really poor and I have struggled with it over that last few years. I keep taking advice and watching tutorials online, but nothing seems to make a difference. I have had 2 Xbox 360's over the last few years and a PS3, my PS3 sometimes has troubles, but no way as much as my Xbox 360. I have had many routers too, trying to find the best ones, spending over �100/$160+. My router (N-Range(always had N-Range routers)/Belkin) is situated in a small office about 25m away from my wireless devices in my house. The other devices work well, just the Xbox 360 has the problem (the New Xbox 360 250GB btw), my previous Xbox 360 (Premium 60GB) had the normal official wireless networking adaptor (not the N-Range one) and that had a improved connection than the new Xbox 360 which I'm currently using, it seemed to be better. I can't move my Xbox 360 nearer the router or the other way round, nor have a wired connection, I've moved my Xbox 360 to the very corner windowsill in my room, hoping for a better connection, but failing. All my other wireless devices, such as my iPhone, PS3, Laptops, Wii, iPod Touch etc work fine, just my Xbox 360. I just don't know what to do any more, the connection's on and off, some times it'll work alright (but not excellent) or really bad. I've had many connection errors (btw wireless connection is around 2/3 bars) on it from MTU errors to just failing to connect to the router. Sometimes in the Internet Connection Test it'll go through the process fine, completing and saying that I can connect to Xbox Live and then saying "Can't Sign In". I know people might not suggest a lot, or be able to fully resolve this problematic and painful issue, but some knowledgeable advice would be greatly appreciated, thanks.