I am a motion graphics artist and work with thousands of high resolution still’s in a single timeline daily and have never had any problems so I would say that you should never have a problem with having too many stills in a project or timeline. Also since it occurs during rendering also I would have to say that it is not a problem with the computer at all. Definitly not any sort of fragmentation issue or lack of ram as these things would just slow the render down not affect its quality. (In fact defraging is not near as important as people make it out to be. While you do need to defrag every once and a while your better off doing it less than more just because of the wear and tear on the drive. The first thing I would do is try a different project to ensure that it is not a setting error or a currupt file. If this does not work then reinstall the software. Being that this happens during rendering it almost has to be a software problem.
