Mac Pro 5,1 weird RAM issue

I have a Mac Pro 5,1 with a pair 8GB sticks of RAM (16GB total). I decided to add another pair of 8GB sticks. When I installed the RAM in slots 1, 2, 5, and 6, the red light for slot 6 on the riser card lit up and the system only saw 24GB total.

When I installed the RAM in slots 1, 2, 3, and 5, all 32GB was seen and no red lights.

When I installed the RAM in slots 1, 3, 5, and 7, all 32GB was see and no red lights.

When I installed six 1GB sticks in slots 1, 2, 3, 5, 6, and 7, all 6GB was seen and no red lights.

When I Installed four 1GB sticks in slots 1, 2, 5, and 6, all 4GB was seen and no red lights.

Is this behavior normal? Have any other Mac Pro 4,1 or 5,1 users experienced this? Thanks.

Upgraded. Thanks for the info. I'll try redoing the upgrade now.

I redid it all and nothing has changed.

Do you mean a bent pin in the CPU socket?

Thanks for the info. I will check when I have a bit more time.

I checked all the pins with a magnifying glass and all looked good. I also swapped the position of the two CPUs. Nothing changed. All symptoms remain the same.

The oddest part is that when I try with 1GB sticks of RAM, everything works as it should.

I am using X5570s. Stepping is SLBF3.

5,1

Originals were E5420s. My assumption was that since 4,1 models could be upgraded to 5,1 models with a firmware flash and were able to run X5570s fine, a machine that started off as a 5,1 would not have problems either. Again, this was just my assumption and I could be way off the mark and totally wrong...

Oops. I meant E5620s

Unfortunately, the original CPUs are long gone. I sold them after installing the X5570s and ended up profiting from the upgrade.

I am beginning to suspect the RAM modules that I have installed. I have four sticks of these Kingstons.

I think I've exhausted all reasonable testing measures and I think I will just leave well enough alone for now. Thanks for all the help.