Mac Pro kernel panic, RAM does not seem to be problem

My Mac Pro (late 2006) now has kernel panics at startup. Problem first happened while computer was in sleep mode, a second kernel panic happened about 10-15 minutes after reboot the first time. Now it happens at startup every time. Zapped PRAM multiple times to no effect. Tested original 10.4 install disk, as well as 10.5 and 10.6 upgrade disks with all external connections removed and from a cold boot. The 10.4 disk causes the kernel panic every time. The 10.5 and 10.6 disks do too, if the C or D key is depressed. The upgrade disks will hang in a gray screen (10.6) or pale blue screen (10.5) with no keys depressed--computer keeps running but fails to boot. Also tested the RAM (4 X 512 MB) in different combinations of one pair in the top riser board (and switched risers, and removed lower board completely). All resulted in kernel panics. Next option?