I've tried it with an animation that's 16 frames long and 360 frames, same thing with both files.
Mac OS X 10.4.10, all the current updates and drivers and such...
Mac Pro Tower, dual Processors 3GHz, 2 Gb RAM, 240 GB free on my start-up drive. Dual monitor: Apple 23" Cinema and 20" Samsung Syncmaster. ATI Radeon X1900 with 512Mb or VRam. Capicorn. Likes long walks with that special peripheral on moonlit nights...opps, wrong forum

The Work-around that seems to prevent this is to have a corner of one of my monitors set up to prevent the Screen saver from activation. The screen saver is the one that comes with the OS, set it up to show my Itunes album covers.
Any idea what could be causing this, as long as I save before playing the animation, I don't lose any work and this isn't a showstopper for me, but it is a speed bump of sorts.