I have one ECU that does this. And in my case it's the ECU. It does everything else normal, but will not steady cruise, it has an SNS STG4 chip in it exact same as my good ECU in my car. So it works perfectly fine under any boost, and at idle. But like cruising on the highway on 110kph the car will stutter and shake horribly. If I go WOT or close the throttle for a second (touching either throttle switch), then it cruises again fine for 30seconds or so and then it starts stuttering again.
I think it happened when my O2 wires rubbed through and it damaged O2 circuitry in the ECU. So it's not impossible welding on the car body damaged the ECU.
I keep that old ECU it because it would still make a good Race ECU, you never hold maintenance throttle for more then 30 seconds in a road race or auto-x anyways.