If memtest shows a specific memory region as failing, swap out sticks to check which it is, and buy a new one. (Or if you're on a tight budget, you can disable that region with kernel boot options.)
If memtest gives errors in lots of places, might be a bad overclock. Loosen timings or give it more voltage.
I would also recommend running it on one memory slot only (do this one by one).
Recently it happened that even a multi-day memtest run did not catch the defect in one of my ram slots.
But when I emptied it, all the gpu driver problems immediately stopped.
The symptom was a crash followed by garbage on screen and a "zzzz" sound. Sometimes followed by a shutdown.
Honestly, I'd always do that as the first option. In most cases you can still get years of life out of that stick of ram (but do check regularly, like after a week and then double the interval every time if it didn't get worse).