ok, at this point I assume you have checked each memory module in single mode.
If not, you should catch up on doing it because the next step is the last resort.
Apart from that, the RAM specs look fine.
So we need to find out if the memory subsystem is working properly...
first, remove the external power from the unit.
Remove all expansion cards, memory modules and connectors from the system board, then take the whole assembly out.
No need to dismount it, just grab the tray and pull it out.
Remove the battery, this will clear all BIOS settings including system time.
This is a brute method, but sometimes BIOS configuration data can become corrupted.
Look if there is any dust in the memory slot and clean it off.
Put the DIMMs into the memory slots and make sure they are seated properly.
This should be easy when you have the board right in front of you.
Then put the battery back in its socket, put the system board assembly back into the case and reconnect everything.
Install the expansion cards, connect the keyboard and monitor.
As soon as the power supply is fed with AC voltage, the system will fire up and do an extended power on self test.
Two error messages will appear soon, reporting that the system time is not set and the system settings are invalid.
Nothing to worry about, just don't forget to go into F10 Setup later to reconfigure the BIOS.
If the memory related errors still come up, the memory subsystem is very likely defective.
The only way to fix this is to replace the system board.