First make sure the cds your using arent scratched or dirty.
Then check the lens to your dreamcast for and dirt or dust.
Check the lid switch some dc's switches wear out press down on it and wiggle
If they all check out alright then it sounds like its time to recalibrate your lazer, although there are drawbacks to doing it. It may shorten the the life of your gd-rom drive and if done wrong you can ruin it, but its very normal for an aging dreamcast to need a tune-up.
Heres a tutorial http://consolevision.com/members/fac...sercalib.shtml