DELL really needs to help out here. The machine is a little older (late 2009) and may not be on it's first owner but it would seem a common trend that the Bios is not allowing the full memory (I am getting 16 out of 32Gb's) of memory to operate.

I have tried V 10A and 11A to see if this will stop the issue. These are still today fantastic machines and bell built as I am sure you would all agree. This sort of issue damages the robust reputation of a quality manufacturer.

I have read lots of excuses for this computer to not be working correctly so lets all put a detailed account on here and help each other and hopefully DELL help us....

I have the DELL T5400 running 2 x 3.16Ghz 4 core xenon processors and a GTX 970 GPU
8X MICRON 4 GB 2RX4 PC2-5300F-555-12-E0 4GB DDR2 667 CL5 ECC
BIOS 11A sees 16 GB's of memory and throws a memory error on boot up.
I ran Dell diagnostics from the web and no issue was found. (all green ticks)
Press F1 on bios boot and go into Windows 10 Pro 64.
The OS works well but is working in 16GB's of memory and 16 GB's not used!
Reset the Bios in the Bios controls .... No affect.
I also tried Installing Bios 10A and restarted this also has no affect.

History
I was running 8 x 2 GB (two different manufacturers) and this gave me all 16GB so all the slots are working correctly.

This to me looks to be a BIOS issue.
If you have the same issue please post in detail what you have and what it is doing. this is the way to get issues fixed.
If you have the fix please share it. it would appear that there are many people that need your help.

As a last note ....Come on DELL do what you say on your web site and "Put the customers first". I am speaking to you as a second customer, as I am sure many people with the issues are, we are the reason that your machines retain value and indirectly bolster your sales and reputation as a top manufacturer of quality.