More Robotron problems

I've had a long Christmas holiday, and as we had visitors, arcade related activites had to wait.

I was eager to get back into the game room, and especially to test Robotron with the newly acquired 4164 ram chips. I inserted them just before the holiday, but I still got an error on chip 3 in bank 2. A couple of days later, I realized I should have measured the voltages on the pins of that particular chip, but Christmas came and went before I had the chance,

While measuring, the 1-2-3 error suddenly went away, and the graphics on the monitor changed dramatically. Everything from a uniform light purple color to what looked like changing graphics/logos with heavy disturbances. I can't say that I recognized the Robotron graphics, but it was still nice to see something else for a change.

I turned off the cabinet and turned it back on, and disaster struck! The  numerical error display was suddenly just flashing, not really displaying anything else than a 0. The monitor went back to the pre-boot garbage, but with some extra disturbances added.

What worries me is that the 1-2-3 display comes back if I press lightly on any of the ram chips in the lower right corner.

Video (sorry about the terrible quality):

You may not see it, but the display changes back to 1-2-3 when I press the ram chips lightly. The monitor picture is not as bad as it looks. The diagonal lines aren't there, and it's basically more or less the same garbage as Robotron has before it starts. 

I am desperately trying to find the source of this error, but I don't really know where to start. It looks like there is a bad contact somewhere, but it's difficult to pin-point it.

Any hints or tips will be greatly appreciated.