I'm also going with cables - they are 20 years old and this type of cable gets pretty flaky when unplugged/plugged a lot.

A new set isn't expensive - I would start there.

Agreed , First thing I tried was a full set of ribbons from a known working machine,also a display driver board,then a display,All to no avail

So Swapped Mpu boards and Fault was transferred to new Machine, And Faulty one works fine with the other Machines Mpu..

So its Pointing towards THe Mpu....

So Far I have changed U 1-2-3 which are a pair of 74LS244 and 74LS245, and Reseated Processor chip and Asic and Game roms been swapped a few times..