HelP!! Why won't my Dreamcast play a lot of games?

[fernicus]

I just bought my Dreamcast used and tried playing a bunch of backups I bought from a guy. This guy has alot of stuff - Playstation, Playstation 2, and DC so I know that his stuff is good. My problem is that 4 or so of the games don't play at all on my system. I tried the bootcd but nothing seems to work. I've brought the games back thinking they were bad copies and tried replacements but they didn't work either. Could my bootdisc be outdated? I noticed the bootdisc has a version number, do I need a newer one? Is there something wrong with my Dreamcast that others have also encountered? I have a legitimate game also called Ripping Riders, I think, a snowboarding game and it too seizes up in the middle of the game.HELP!!!