Memory Issue

[Co-nan]

I'm hoping someone out there can help or enlighten me, 

I have a HP Elite 8100 which decided to play up, Firstly it wouldn't boot (no display) and then the PSU fan went fast and noised, I checked the PSU with a DVM and all seemed ok, at this point I assumed it was the motherboard and replaced it, nope.. the fan still went rapid and no boot up, I then bought a new PSU and still the problem occurred. 

I when restored the original motherboard and still no joy ...

So now I thought I'll try with just one of the 4x4gb Ram stick and it booted normally!! ... I continued to try with a second 4Gb RAM stick and again it booted ok!!, now the issue occurs when I try to start the PC with 3 or 4 RAM sticks.

I have tried all 4 of the RAM singerly and they all work fine ...so no faulty components.

Just to add I had been running this machine for around 2 years with 16gb of RAM (4X4gb) with OS Windows 10 (64), so why now after 2 years would it decided it doesn't like this.

OS is all up to date and so is the BOIS (no upgrades needed)

HELP!!!

[Co-nan]

Just to add additional information, the 4x 4gb of RAM I'm using are ....

Samsung  4GB 2Rx8 PC3-12800U-09-11-81 (M378B5273DH0-CH9 1430)