About ICs - am I right in thinking U3A would be the side of the IC with the little circular "dimple" and U3B the other side?
The general-case answer is not necessarily. It depends on what the IC is and how the person (or software) who drew the schematic and laid out the PCB assigned the pin numbers and part designations. The only way you can tell for sure is to look at the schematic and see which pin numbers are used for sections A and B. Pin 1 is near the dimple and the numbers run along the edges. If it's an 8-pin DIP then pin 8 is near pin 1, on the opposite side.
However, in the specific case of U3 in the 420, it appears from the schematic that A is on the dimple side, since U3A uses pins 1-3.
If it's an intermittent problem--that is, it changed when you did something mechanical (inserting the cable)--then odds are good it's a bad solder joint somewhere. I'd probably start by going over the entire PCB solder side with a magnifying glass. Yes, that means you'll have to mechanically disassemble the module, but that's why I go over the entire PCB with a magnifying glass before I assemble it further. :-)
Good luck!
--Adam