Precision Tower 3420 Failing DIMM slots on motherboard?

Hi 
I have a Dell Precision Tower Model 3420 
Processor:  Intel(R) Core(TM) i5-6600 CPU @ 3.30GHz
Memory: 2x8GB Corsair Vengeance LPX DDR4 2666 MHz DIMMs
Date of manufacture March 2016

It has been working reliably without any issues until December 2023 when it was starting to become unstable.  I tracked this down to a memory fault which seemed to be getting progressively worse over time
By substation and movement of the DIMMs through slots 1 to 4 I worked out that DIMM slot #2 was faulty.  In fact with a DIMM memory module in this slot the PC would not even post only giving the 2-7 flash code on the power switch.  The slots and memory were clean, no dust.

I moved the two DIMMs from slots 1&2 up to slots 3&4 and all was well.  I made a note that slot #2 was faulty. 
Fast forward to February 2024 and I am having the same issue, this time slot #4 seems to have become faulty as well as #2 interesting that they are in the same bank/upper block!
The machine is now running ok with slots 1&3 populated and I am getting 16GB all be it single channel which is not ideal but better than nothing.

I am posting this here to ask if anyone else has had a similar problem and if so what the root cause was.  My concern is the motherboard memory controller is staring to break down and will continue to do so eventually becoming unusable.

I don't have any spare DDR4 DIMMs so can't rule out if they are part of the problem.

Any help would be appreciated as I like this PC an would like to continue using it.

Cheers