so when you put a substitute mpu board in your machine and you do a display test you dont get an error in display test.
and you put the suspect mpu board in a different machine and do the display test you get the error in the different machine?
if this is the case then the fault has to be on the mpu board.
Yep this is the case..... 
before replacing the ram chip on the mpu board if you haven't done so already i would be beeping out all the address and data lines between U1, 2, 3 and the cpu chip.
I would find it strange that you would have only a display fault if you had a faulty ram chip on the mpu board, i would expect lamp, switch , solenoid and flasher problems as well.
but anything is possible
I've previously socketed U1-2-3 and ensured all pins went to where they were supposed to.
Also had a Cro on all the data and address lines and seem to have activity on all..
Weather it's correct is beyond my testing ability..
It has had the odd random coil fire and flipper flutter in WW with this Mpu board installed..
