Since core
memory reads are destructive, we can eliminate some core issues if you
say that core can be read repeatedly without it's contents changing. It
could be a problem with the path between the switch register and the
memory data bus.
But he wrote that the data displays correctly. The 8/e display
shows the
state of the memory data bus. My first idea would be to check for bad
contacts. And think about the memory control.
And? Can you read out memory? Does it change?