If fishtales display does not work in the 911 machine, I reckon the ribbon cable could still be suspect in 911 and u have a intermittant ribbon cable issue....fishy dmd not working in 911 proves u have a ribbon cable and/or probable power supply problem...

I had a machine which ws doing similar stuff all round....it was the ribbon cable...because the power on one machine to the dmd was SLIGHTLY higher and with the slight voltage increase the suspect ribbon cable would work, but with a slightly less voltage it wouldnt work...it just might be easier to replace both the ribbon cable on 911 and look at the power/voltage supply rebuild as well... sounds like 2 small issuew working together to me with everything u have written...fault finding, fun isnt it....not.lol