Some things which might be worth looking at?
- Have you tried to run the memory diagnostics from the BIOS with all four memory simms (/dimms) in place?
- Do you need to tell the BIOS that you have added memory?
- Are there settings in the BIOS to do with mapping memory used by any integrated (motherboard) graphics processor (even if you're not using it...)?
- can the motherboard definitely support support 4*8GB chips?
Good luck
Iain