Car runs rough only under heavy load = high throttle + low RPM. The car idles great, and is smooth under low loads as well as full throttle, high RPM.
This is a bucking, jerking kind of torque loss that is persistent until engine RPM increases or I let off the throttle. Not like a momentary hesitation. Passing without downshifting is pretty much impossible.
The car recently passed smog test and has never shown a check engine light.
Under the hood, spark plug wires and contacts look fine. Intake system looks in good condition and solidly connected. I have not looked inside any of the intake components.
Background info
The problem started after a broken timing belt and water pump were replaced. I brought it in when I noticed the problem after the repair and they recognized the issue and checked things but said all was well with their work. From their viewpoint the car was brought in not running and they could not be sure this was not a preexisting condition.