OBD port not connecting to ECU

The previous owner tried to piggyback off OBD power line, clipped through the cable, leaving 1/8" stub. Replaced OBD port, re-soldered all wires, have power, but got message that it is not connecting to ECU. (OBD program (Torque) sees chip code for OBD reader, shows power to reader.) What else could be wrong?
Additionally, since last oil change (less than 1,000 miles since), engine is running rough at idle (shudders), hesitates on acceleration, and periodically Traction Control and Stabilitrak error lights come on, but car handles fine, corners normally & brakes well, runs fine at highway speeds. Air filter is clean. EGR looks brand new. Plugs replaced approximately 6 months ago, show no unusual deposits or pitting, gap is set correctly. I understand the rough idle could be a number of things, including PCR valve, idle control valve, etc. Someone suggested rough acceleration and rough idle could be rear differential slipping, although I'm not seeing issues with steering, wheels squealing when turning, etc, which I understand is common with differential issues.
So, without access to engine codes, how do I determine cause of issues, or better yet, how do I determine why OBD port is not communicating with ECU. Is the ECU causing all these issues? Thanks!