Check the Math proms......when one or more fail they can cause Matrix errors, they are located at 7h,7j,7k and 7L. You would need a working board to swap them out.
It could be something else though as the matrix section is pretty complicated.
Mine had this issue last year, and it was the chip at 7A (Alex Yeckley fixed it for me as I tried but couldn't nail it down). Try looking at the chips in this "Register" section of the board (chips at 7A - 7D) with a logic probe or o-scope.