Recently did a lot to refresh the engine on my 98 Civic CX, 5 speed. I've replaced all the sensors, new IACV (twice) and new motor mounts. Replaced head gasket, had head machined for flatness, etc...

Issue I'm having is sometimes, after driving the car for a several miles, it will throw a P0505 code and the car will begin jerking when pressing the gas pedal or releasing the gas pedal. If I clear the code, it acts completely normal. The time it takes for it to start acting up varies greatly. Sometimes I will drive it for 20 miles and never skips a beat, then other times it happens after only a few miles into the drive...I've checked for vacuum leaks, none that I can see even after smoke test.

Any ideas on what to check on next that results in this behavior?