(Processor Memory Maintenance?) Memory Stack adjustment- something like - setting the control to the midpoint between failures.
I can remember running the extended memory checkerboard diagnostic and finding the stack would pass without any errors in 3 of the 8 positions, I set the switch in the middle.
A better example would be in the 1st position - failed, 2nd position - test runs, halts occasionally with an error, 3rd / 4th / 5th - passes for an extended period, no errors, 6th position same as 2nd, 7th fails. The best position for this would be 4.