I had the same kind of problem with one , except that it was only ghosting on 1 of the displays and the others were fine.
It turned out to be one of the smaller chips on the MPU board but i can't say from memory which one.
I'm sure it was one of the 4011's or 4049 but i'm unsure.

Thanks Ken. I know i replaced a 4011 on the board, there is also a 4049 that is part of the blanking circuit which i dont think i've changed, will try that next time i'm at Ians house.