Land Rover Repair: 1994 Defender 90 stumbles

Question
i have a 1994 NAS Defender 90, 145,000 miles. It will occasionally run rough and stumble from start up, as if cylinders are not firing. No codes. If I disconnect/reconnect serial port (with ignition off) it seems to temporally cure the problem. Display shows 02 code, so box is apprently working. it will also stumble at highway speeds in fifth. No code or Check Engine light. Defender will sometimes show a Check Engine light and be running fine. O2 sensors are 8 years old and have 10,000 miles on them. ECU replaced about 5 years ago with a used unit, about 5,000 miles. Web surfing shows anything from bad ECU or coil to bad ground.

thanks for your time and looking forward to your thoughts.

Mike

Answer
The serial port to box connection should not affect the running of the car.  I don't know if that's clue or coincidence.  Your car is too old for any sophisticated diagnostics and it's old enough that wire harness corrosion and random failures become likely.  There no no single thing I could point to.  In many cases the parts in your truck are bet tested by substitution, which we often do.  We can also use the Land Rover test system for your truck to look at live data - such as it is - from the ecu and possibly see clues.  

Most of the testing of a truck like yours is wire by wire with a mater, rather than by expensive scanner like on a new car