Inspiron 23 AIO - Only boots AFTER memory size change ???

Strangest thing - this Inspiron 23 AIO wouldn't boot, not even to the POST. Power light comes on, stays solid white, but nothing on the screen. At one point I took out one of the two 2GB RAMs. Voila - it booted to Windows! I figured that was it - bad RAM stick. So I put the old one back in to prove it - boots OK again! Put the cover back on, didn't boot. Pulled a RAM stick again - boots!.

To make a long story short, the machine will boot exactly ONCE whenever the RAM size CHANGES. Then won't boot again until I go from 2GB to 4GB, or from 4GB to 2GB. Doesn't matter which stick goes in or out. Tried some other sticks - same result. Runs all the F5 "thorough test" diagnostics just fine. When it does boot into Windows, all's well until the next reboot. Tried defaulting the BIOS, no change. Power supply voltages look fine. Fan's OK. Tried to just power-down for a while - doesn't work.

It will boot fine exactly ONCE whenever i CHANGE the RAM size. Second boot, it just sits there, nothing on the screen, no post, no beeps, power light is solid white.

Never saw anything like it. Google hasn't found anything, but it's a tough question to ask. Any ideas about the cause, solution, or other tests appreciated.

Thank you.