IMac 27" Boot issue

Question

I have a late '09 iMac 27" Quad Core i5. Almost every time I go to boot, I hear the hard drive spin up and then nothing, just a black screen. No beeps, just black and near silence. it does this about 95% of the time. Here's the kicker, if I take RAM out, or put some in, or  re-seat it, it'll boot OK, and then run fine. The problem initially started when I swapped some RAM trying to resolve some kernel panic issues (that turned out to be old, conflicting Digidesign drivers). When I put the new RAM in, it booted fine. When I put the RAM cover on, it wouldn't boot. Take the cover off again, booted no problem. Then a few boots later, nothing again.
I've ruled out bad RAM, as I've used all new RAM and still the same issue, and putting the old RAM back again will sometimes allow it to boot, until next time, then taking some RAM out will boot it again. Sometimes unplugging everything form the back will allow it to boot, sometimes it won't. Once it boots, it runs nicely. If what I've said here doesn't make any sense, then it means I've explained it properly! Here's what I've tried already:
All new RAM
Run Hardware Test (overnight, looping and extended test mode, no issues)
All external devices unplugged
All software updates applied, including 3rd party
Verify Disk
Zap PRAM
Reset the SMC
A service centre I called said it may be the memory management controller, but that's part of the logic board and that's a last resort financially. It's also hard to see how they'll know for sure because of the nature of the issue. It's so consistent in it's non-booting, but what I have to do to get it to boot again is not consistent, and just doesn't make any sense to me.
Anyone have any ideas? Thanks in advance...

Answer 1

Thanks for the reply. It seems (at least for now) that my solution is the same as yours. Very bizarre. I ran diagnostics on the RAM and that came back without error. Currently I have no issues but obviously it would be a pain if I had to shuffle the RAM around every time I wanted to start the Mac from cold. Just in case it happens again I haven't yet replaced the cover to the RAM slots. Seriously though, if the Harware guys are watching these forums: This upgrade should be at least as reliable and almost as simple as inserting a usb stick. There are too many people who have RAM upgrade problems.
Thanks again. Very much appreciated.