For a PROPER I have no way of determining whether the file on disk is a PROPER or not, so I rely on the fact that the NZB for a proper has already been imported AND that particular episode is not in the queue (since it may have already been added, but the NZB has not been loaded). The only time I delete an episode from disk for replacement with a PROPER is when the config option "downloadPropers" is true, and the above conditions are met (not in queue AND not already imported).
When SABSync comes across a PROPER (and downloadPropers = true) it will look for the item in the Queue and check for a matching NZB file, if neither are found it will set a flag to delete the PROPER, once the file is deleted it will begin the normal process to determine if that episode is still wanted, which unless that season is ignored, or the download quality is not wanted it will download that episode (re-thinking this, I will need to move these checks to above the PROPER check, since it would be possibly to delete a file and then later determine that the season is being ignored or the quality is not wanted). This should not apply to your situation as it appears you are using an XviD RSS feed only.
The only way I can see a PROPER causing an issue would be if you previously caught it downloading an episode you already had again (the PROPER) and then you canceled the download, since it would have deleted the episode on disk to make room for the PROPER. Next time SABSync ran it would find the PROPER and skip it because it was already in the imported NZB folder, but it would try to download the next episode of that name found if it had not already been imported, because the episode was no longer on disk.
Like I said above, SABSync will only delete the episode on disk if the PROPER NZB was not imported, so when you are seeing it skip a PROPER, but trying to download a non-PROPER it could only be caused by the above issue. If you downloaded a PROPER and then later deleted the imported NZB, it would re-download it again because it thinks you don't have the PROPER on disk.
-Markus



