Had 512mb ram installed in each slot and one day just stopped reading one of the slots. Checked BIOS and only detected 512mb, reset defaults and same issue. Checked RAM and both are fine. removed the 512mb ram and installed one 1gb and it only works on one slot only. Have cpuid installed and it shows the ram installed in each of slots fine
Computers & Internet - Gigabyte - 7VM400M-RZ (GA-7VM400M-RZ) Motherboard
Answers & Comments
Your motherboard is quite old. It could be the RAM slot is simply dirty(use canned air), but if that's the case, the PC doesn't boot. The memory controller could be dying(replace motherboard or upgrade system). There could be other issues but it sounds like your memory itself is fine.
You could try reinstalling the BIOS, but because of the age of your computer, you shouldn't spend any money trying to get it fixed. It would probably be cheaper just to get a compatible used motherboard on eBay as replacement. The better long term financial decision would have to be building either a used parts PC (because CPUs have barely gotten better in the last 5 years) or a brand new system.
This isn't very much information to draw on, so if you can update your information, I can help more.