How many RAM slots do you have? and, the RAM stick that you have removed, how far it was seated from the CPU fan?
If it is the nearest one, the hot air blows from the CPU fan may cause the issue.
Test your RAM modules for possible errors. How to Test and Diagnose RAM Issues with Memtest86+
Run memtest for at least 8 passes, preferably overnight, per RAM module per slot. In first run, run it will all RAM modules. Next, one module in one slot, then another module in that very slot; and thus, test it in all possible combinations of RAM modules and slots. It will take a long time, but it will rule it out clearly if any particular RAM stick, or any particular RAM slot is under problem. Or it is just a problem I described earlier in my post, hot air issue.