the reason why these things happen - is that when play audio is used-
the real movie frame rate is used-

the real movie frame rate might be exactly 30fps or 25fps -

DOES NOT match the vga or dvi timings-

vga/dvi timings are different at different screen sizes-

they are 60.02 or 75.03 or 59.94 - very rarely exactly 75Hz or 60Hz

so your movie is playing back NOT locked to the screen outputs-

There arent any drop frames - or any stuttering - the frames are not being output locked to the screen output rate - so at certain times - usually every 30 secs dont have any chance to get displayed- as the movie playback goes out of sync with the screen rate


you want to do audio and dropped frames are critical-

you need to use single or dual output - at exact same size-
do not use triple heads-
do not mix monitor resolutions-

use 1920x1080 or 1280x720 or 800x600 similar-

run catalyst playback at 60fps or 50fps or 75fps -

this give more of a chance for frames which dont line up with the screen refresh rate to get displayed.