Two of the screws on a w126 (if that is what you have) are ground points for the gauges.
They send the ground to the center connector (rectangle one) and then, through the speedo, it branchs out via some metal strips which are part of the whole speedo unit. Check these points where they contact the other two circuit boards (left and right).

Edit: Just noticed you have a w124 body? I think its very similar on that one too...worth checking atleast as you disturbed/changed the unit which could be causing these problems