Precision 3630 - Ram no longer working although ram works in another machine?

Hi, 

I have dusted off a Precision 3630 tower.  The tower no longer boots and I get a ram error code lights.  The tower's configuration is untouched from the last time I used it (couple of years ago) and has the factory fitted 4 sticks of 8gb ram.

I went through these attempts to resolve - removed all ram and replaced each in turn to slot 1, no difference.  repeated but populating slots 1 and 3 but again to no joy.  I  have cleaned the contacts with an eraser.  I have tried the ram in another machine , which booted fine.

I borrowed some gaming ram and the 3630 booted fine, I was able to update windows and all drivers including firmware to the latest versions.

Removed the gaming ram and replaced with the originals  = back to ram error! Ram works, Computer works just not together although they did???  Just can't figure this one out.

Confused.