Plastic... you've made me think about when I have had problems or success... and I think that I might be in the same, unexplainable, boat.
Whenever I'm dabbling on email or small stuff, the connection is sporadic and frustratingly unresponsive - not all the time, but too often not to notice. But, when I watch my Slingbox (based at my friend's house in the US), its constant streaming (live or recorded TV) seems to steady everything. I remember thinking if the steady stream would slow things down - eg. take up the bandwidth - but have had no issue with too much being downloaded and it actually makes it better for surfing etc....