This can indicate corruption or missing data in the middle of the file. There are programs that will scan the whole file to make sure everything’s there, but most media players won’t bother.
In cases like that I personally use VirtualDub. (I know it’s an AVI tool, but it can read MPEG-1). It will scan the whole MPEG-1 file. If you’re getting an error message (anachronistic timestamps or something like that), then my guess is correct. After the scan the file should play fine in VirtualDub though.