I tried some - no scene just added clouds, 640x480 final.
My pleasure to say I have no problems with that. Very similar to Gill's results:
no clouds 11"
Dark Cumulus (spectral 2) 2'38"
Swirly cumulus (volumetric) 12"
Large cummulus 2 (spectral 1) 47"

My computer is a macbooc pro intel core 2 Duo, 2.16 GHz, 3GB memory
ATY,RadeonX1600

I hope they sort it for you - good luck!

"I paint that which comes from the imagination or from dreams, or from an unconscious drive. I photograph the things that I do not wish to paint, the things which already have an existence."
Man Ray, modernist painter
http://artpearl.redbubble.com/