mencoder causes system freeze/general dvd rip issues
 

Hi all -
Trying to rip dvd's, currently using Arch but I've tried mencoder on at least four other distros, so I suspect this is hardware-related.
On every distro I've tried dvd-rip crashes when ripping with a 'broken pipe' error message. I also try mencoder, and EVERY TIME - on every distro I've tried - it gets to about 6% done then everythign freezes up and I can only switch off the power. I've tried to monitor this with top, and I do get upto 99% cpu usage with mencoder, and a lot of the RAM seems to be eaten up too.
What confuses me is that normal day to day usage is fine - recompiling kernel, playing films and music, web browsing etc, - its only when I try and rip a dvd that this happens. So perhaps this should go in the hardware forum, but I really don't know where to start diagnosing this.
My system is an athlon xp 1800 on a gigabyte ga-7dxe mobo, with 768mb ram. I've ran memtest,which came up clean - and taken out different strips of ram but same thing happens.
Any hints to where I can poke around would be appreciated.