on certain downloads the browser will not download but just says "stopped" in the downloads window. growl update, for example.

Sometimes, for no apparent reason, I cannot download certain things. The "growl" update, for one example and this site http://www.bestshareware.net/downloading/pavtube-mkv-converter-for-mac.htm for another. It begins the download but almost immediately stops and displays "stopped" in the bottom left corner. The download does not show in the "downloads" window. This began (I think) after I had upgraded to FF4 and then decided to go back to 3.6 (which I MUCH prefer). I notice that it also prevents the "download Youtube" add on from working. Again the download begins for a second and then just "stops". But MOST things download just fine. Any ideas?