first of all, look on youtube on how to clean the laser assembly.
(don't do this line til last resort: potentiometer calibration if cleaning fails)
okay, are they retail games? (gdi)
if so, the only real way to test the disks, is to dump them via sd card, or network stream. (there's other mods to do it, expensive and way off track)
if the gdi dumps without fail, it's something related to the software/hardware settings.
if it doesn't, could be a damaged retail, or dirty/screwed lens.
the spot repeating strikes me as possible done in disk?
POSSIBLE, NOT certain.
gdi to sd dumper, get that, and write it to disk (imgburn) and boot it up.
you will need a serial socket to sd card reader to dump retail disks.
there's nw options, searches will help if you have a bb card for the dc, otherwise, sd card reader. (internal or external.)
of course, if you refer to backup games/RIPS, try other images (cdi/iso) and/or a different media.
backup games are out of my realm, retail player only. (working on getting gdi-usb)
if you can get access to another dreamcast, try the disks on that, if the exact same bs happens there, good chance the retail is dead/bad write for backup.
all the best!