Oh, I see. (I thought that the crash occurred only when the video was paused and you tried to do a grab.)
Obviously at least one of the DirectShow filters is not working correctly with those files. This may be caused by the combination of more or less "bad" files and filters.
What is your Windows version? Have you installed any DirectShow filters?
Can you describe the file contents? Do the problem files have anything in common. For instance: are they small, big, DVD quality, created by a digital camera or downloaded, etc?
For checking the file contents you could try MediaInfo. (
http://www.videohelp.com/tools/MediaInfo)
For fixing the unstable behavior you could try the Gabest MPEG splitter from the DirectShow Filter Pack (
http://www.degeelebosch.nl/corone/) and CCCP for installing FFDShow (
http://www.cccp-project.net/)
After installing you would need to configure the FFDShow video decoder to decode MPEG (the "libavcodec" option is recommended here:
http://www.degeelebosch.nl/corone/dsfp/main.htm#ffdshow_config)...
... and set MC to use the installed splitter and video decoder filters for the MPG file type.
Recording file conversion there are ways to demux and remux the contents (i.e. rebuild the container without re-encoding), but actual re-encoding will always affect the quality (more or less). When done correctly the results might be fine.