Tinaw,

You can automate this with the script I wrote to cap Dead Awake. Open the Script Editor (Applications:AppleScript:Script Editor). Copy and paste this into a new script:

set a to 0
repeat (98 * 3) times --98 minute movie, 3 grabs per minute
delay 20 -- wait 20 seconds between screengrabs (3 per minute)
do shell script "screencapture ~/Desktop/grabs/screenshot" & a & ".png"
set a to a + 1
end repeat


You'll need to customize the "repeat (98 * 3) times" line and the "delay 20" line. The delay is how many seconds to wait between taking a grab. The repeat is how long it should run total (length of the movie times the number of grabs per minute). It will save the grabs to a folder on the desktop, and number them sequentially.

To run it, hit play in VLC and click Run in the Script Editor. Hope this helps.