Several problems and solutions that I have learned over the years.
CPU not properly seated will do that. It might see all 16gb of memory but only use a portion of it. When I build a New PC I put the CPU in the socket and clamp it down. Then take it back out and clamp it back down. I'll do this a couple times before I put my thermal paste on and bolt down the cooler.
Lightly press down on the CPU while you lock it down the last time.
AMDs memory controller is on the CPU so if it's not seated correctly it will cause all types of problems. 1st gen socket 939 and AM2 were notorious for not seating the 1st couple times. You could not just pop it in and go unless you were really lucky lol.
Motherboard could have a faulty memory channel. I've had to RMA quite a few mid priced motherboards lately for PCs that I have built for people.
CPU could be faulty but that rarely happens these days.
Memory compatibility should Not be the issue since you are running the correct speed of memory for the motherboard and that memory is 1.2v. If memory voltage is higher than 1.5v you have to set the voltage up manually. You have Bios Version 3803 which is the latest version.
Here is what I would do if you still have problems after Re-Seating the CPU. This is going to take hours but it's the only way to test both the memory and motherboard.
This is your motherboard. I marked Channel 1 in Blue and Channel 2 in Orange.
Step 1. Install 2 memory modules in channel 1 and load into windows and run Windows Memory diagnostics.
Click Start, type mdsched.exe in the Search box, and then press Enter. Choose whether to restart the computer and run the tool immediately or schedule the tool to run at the next restart.
Let it run untill it's completely done. If errors are displayed Replace memory.
Step 2. Pull those out and put the other 2 in channel 1 and test again.
Step 3. If no errors were detected Repeat the tests above on Channel 2
If you get errors in 1 channel and not the other you will have to RMA the motherboard.
I know it sucks testing 4 sticks of memory in 4 different slots because it takes forever but that's the only way to test all 4 memory modules and all 4 memory slots.
Hope this helps.