I might have the same problem. OSX 10.7.4 SABnzbd 0.7.3 and earlier versions.
I do have sometimes the problem that the files I currently download aren't finishing. I couldn't determine any specific files, it doesn't depend on the file size in contrast to the thread creator. When it happens it's in the last megabytes or even kilobytes of the file thats being downloaded, the download rate slowly begins to slow down to a minimum of 1B/s and persists at that rate. At this point the only thing that helps, is to kill the SABnzbd process, a normal shutdown won't do it, because after pressing shutdown in SABnzbd the application doesn't respond to anything. After a restart of SABnzbd the download continues at full speed. This happened to me already in previous versions, can't tell which ones exactly. But in the last one for sure.