I need some advice: I have a system that refuses to run without memory errors and the resulting file corruption has forced me to start replacing components until I get the advertised/expected performance. In this case, the DDR4-3600 (CL18) RAM I purchased cannot get through Memtest86 (Test7) without a ton of address errors.

Setup1
Ryzen 5 5600X (OEM tray CPU)
MSI B550M PRO VC WIFI: BIOS is dated 9/25/25
KLEVV DDR4-3600 (CL18) : QVL certification confirmed
Using the XMP profile 1 option (CL 18-20-20-40, 1.35VDC)

After loading Fedora 43 Workstation and seeing some odd pauses I tried to install Steam and this is when I realized I had some data corruption going via the Terminal stream. Immediately researched and tried to dial down the speed to see if my CPU's memory controller just couldn't handle the 3600 speed. Tried 3200 and adjusted the timings down to a standard set that were more appropriate for that speed but then I just got Memtest errors almost immediately (Test 2, 3, 4) so I manually aborted the test.
However, if I default back to the auto timings (DDR4-2667 @ 1.20VDC), the whole system passes all of the tests and runs perfectly fine.

As a result I performed the following action:
Replaced the Memory sticks with 2 of the same type. No change in test results at either speed.

Online research suggested the CPU/memory controller was most likely the cause so I replaced the CPU with a newer (retail) version and B2 stepping in the hopes it would perform better (see setup 2 below)

Setup2
Replaced the CPU with a Ryzen 5 5600XT (Retail Box CPU)
MSI B550M PRO VC WIFI (same Mobo)
Used the replacement set of KLEVV DDR4-3600 (CL18)
Using the XMP profile 1 option (CL 18-20-20-40, 1.35VDC)

Results were exactly the same with Test 7 being the failure point using the XMP profile and only the default settings (2667 M/T and auto timings) worked with no issues. I also tried other DDR speeds like 3400, 3200, and 3000 with suggested relaxed timings appropriate for each speed and a voltage boost to 1.35 VDC. Tweaking the RAM voltage up didn't seem to make any difference.

Apologies for the long read so far but now I am at a crossroads with this machine.
I have tightened up the DRAM timings to (CL14-16-16-32) at 2667M/T and 1.20VDC and it runs error free and passes Memtest and stress-ng tests in the OS. I have also been able to get really decent gaming performance and no more corrupted files or random crashing using Steam.

So it doesn't seem to be the CPU or the RAM and the voltages seem OK from the PSU. Should I tear the whole thing apart and replace the motherboard or just stick with what I have since it works (albeit at a lower speed than advertised)?
This is one build that has really stumped me.
Thanks for reading.