Cant change projectmix buffer setting in Cubase 5

Hi, When I change the Projectmix buffer settings to something higher Cubase puts it back to 256 as soon as the project starts to play. Ive tried closing Cubase and then changing the buffer settings from the m-audio FW control panel and re-opening Cubase but it defaults back to 256. anybody know how to fix this?

Cheers

Dave