Exact Ae version please and Os details
It is quite RAM intensive and I have to go easy when I use it, small changes, save often but as I say I have done this exact same activity about a year ago, Impot film, duplicate, line up, mask, render...done.
Your version of AE is old. It can't deal with more recent codecs. You'll have to transcode. I recommend quicktime movies in the Photo JPEG, PNG, JPEG 2000 or animation codecs.
David thank you, that sounds like a plausible answer.
May I ask can you recommend software for converting these files, ideally a free download ?
Thank you again