Have you tried to run it with just one of the memory modules? With other memory modules?

I haven't run that test yet; I haven't had the time.

I'll also try those modules in another motherboard. However, I suspect the problem might lie elsewhere.

I believe the MemTest86 test is much more rigorous than the HIMEM test; the latter relies on the BIOS and other factors, whereas the former accesses the memory directly.

I think it's a setup issue. That’s why I’m asking if anyone else has encountered this problem and how they fixed it.

I see broken images in the initial post. Cant see them.

Memtest pass points to something else, maybe the cache, cache timings? Not sure if memtest disables cache, but it must work in a way that avoids cache hit

Start with the basics. First, check all the contacts on SIMM slots carefully for missing or bent pins, then clean the contacts on both the SIMM slots and the RAMs with a contact cleaner or IPA.

HIMEM reports errors no matter if RAM or Cache (SRAM) causes them. In many cases an "ageing" TAG-RAM is the culprit.

If both sticks pass HIMEM when installed on their own (single stick), it's possible RAM or cache timing is too tight. Windows 95/98 loading exposes some system instability.