Use either
Memtest86+ - Advanced Memory Diagnostic Tool Memtest86+
or
MemTest: RAM reliability tester for Windows for memtest from windows
HCI memtest is high accurate.
Since that one memory slot is causing the errors on its own, with 4 sticks, etc etc, and only randomly works / no works
Everything your stating is pointing to bad memory slot, but on the other hand, 4 memory sticks , when all memory slots work alone, could also not work. This is when an increase to nb voltage can help.