My Quadra 605 is having this weird problem. It started when I finally got the correct amount of VRAM (matched pair) in this machine and every second or third boot the computer would turn on but would never display anything. The hard disk would run and load Mac OS just fine. I tried reseting the PRAM and replacing the battery. I had a ColorSync 17" attached. I had OS 8.1 installed with 20MB RAM. I installed Timbuktu so I could remote controll the machine. This is how I knew it booted because I could remote login and use it fine.
Now, I replaced the original LC040 CPU and installed a slimline heatsink and it will boot after I clear the PRAM, but after the smiley mac is seen., the computer reboots and loads but all is black. Now it won't show an image period.
The VRAM is 2 pairs of 256KB. Could it be bad VRAM chips or a faulty CPU? I doubt the CPU is bad because the Mac shouldn't boot if the CPU fails it's own internal self-test. So maybe the VRAM is bad or the logic board needs replacement? I'm sure 512KB is enough to drive a 17" ColorSync.