Does it crash when you use a production codec instead of long-gop compression codecs?
... I use Keylight in all of my projects.
What kind of footage are you keying? Media Container & codec? Personally, I know that if I use highly-compressed footage, I'm in for a crap shoot. Better to transcode right off the bat.
What is the exact source media? Which camera did it come from? Can you give us details about the computer you are using?