I just noticed pretty much the same problem the other day. If I do a hardware render it's fine. If I do software however it falls over after between 6 and 10 frames. Tried reading the mayaRenderlog.txt file and it says: "Error: error reading scan line:"

I'm loath to do it, but I guess it's time to start nuking things in the prefs directory. Expect a response if I suss it.
-S