Mac Pro won't start from DVD, crashes while trying
Assuming your disc is clean and damage free it looks like your optical drive may be at fault. If your disc does have any obvious defects (check for scratches etc. on the label side, these can cause totally unrecoverable data loss) you know where the fault lies. Has the drive had any sort of regular use or is it a while since you used it?

If you have access to another Mac you will be able to copy the 10.6 DVD onto an 8GB USB stick using Disk Utility's Restore function, which will then boot the Mac - if the optical drive is at fault.