I'm facing a Parity Error problem on my new 486er DOS Retro PC. Every now and then a running software crashes with a Parity Error output on screen. This happens for different software so I think the source is one of my 4 2 MB RAM modules.
I've already tried to proof this with a copy of memtest86 on a bootable floppy disk, but this program also crashes on startup and creates an endless boot loop. It shows a menu and a blue screen is visible afterwards for 2 seconds. There is a debug option to run it step by step which only prints out 2 steps before reboot.
What are my options? How did someone in the early 90s investigates in such a problem? How to detect defect RAM on DOS?