I had a similar problem with my DDR2 value RAM from Buffalo.
At first it began like once a week... I turn on the PC and just a black screen pops but no beep. The solution found was turning the PC off with the PSU button, and do this some 3 times so it could boot.
A month later I had to do this every time i turn on the computer and i ran Memtest86 for 2hours and no problems detected (4 passes if i'm not wrong).
Finally, even without a confirmation if it was the RAM, i bought a new 2x2 kit and the computer will now boot correctly.
I recommend you to borrow a DIM from a friend/neighbor and test it out if the rubber doesn't work.