New logic board, still boot problems with Turbo Fans

Question

I bought a 2Ghz dual G5 from a guy on craigslist and he told me that the Geniuses at the Apple Store said it had a bad logic board. So I bought a brand new logic board from DVwarehouse.com, and installed it along with a new set of matched RAM from Crucial, a new HD, new PRAM battery, new video card and a new superdrive. The only thing original are the processors and power supply (and fans, of course).
When I start it up, after the chime it quickly goes into "turbo-fan mode", which I have read that means the Op Sys didn't load correctly. Many restarts later, I was able to reset the nvram in the Open Firmware, but that didn't seem to help at all. I also pressed the SMU button (according to Apple's directions), but that didn't help much either.
I've had a few successful boots, but always with the fans going full blast. I though that once I got Leopard loaded onto the HD, then it would calm down with a software update. But before I ever get that far, the G5 hangs. Sometimes the G5 will boot and access the HD but hangs soon after. Sometimes I can get it to boot off the Leopard DVD. But most of the time it just boots and sits there slowly spinning up the fans into turbo mode.
The few times it has booted to access the HD or DVD, the fans go turbo right away. Other times it will boot with a chime, but then it won't access the HD or DVD and I get noting on the display.
I should say that this behavior is almost identical to how it was acting before I replaced the logic board. But how can that be? It should be a new computer, unless the problem is in the CPUs. Is that very likely? How would I find out if the CPUs are bad?
I've been reading alot about the problems I'm having with this G5 and what I've been having seems to be very common. But I've run out of fixes. Is there anything I can do to fix this machine or is it just money down the drain?

Answer 1

my iMac 20" G5 experience since December 2004, even though I love this machine,it has gone through 3 power supply (661-3350) replacements and 3 logic board replacement since late 2004. Pretty crazy for a single machine but I still love it. The last logic board replacement did NOT fix my boot up problem, as I expected, and the genius finally agreed to replace the power supply and that fixed it. SO: My recommendation to you is "replaced the power supply" here is one place you can get one: http://www.dvwarehouse.com/Power-supply-180W-for-iMac-G5-20--100v-661-3350---614 -0353-----NEW-p-25613.html