GEN 3 Liberty jerk when accelerate from light throttle

[winabbey]

I have a 2000 Subaru Liberty RX manual that has an annoying unresolved issue.

When cruising along on light throttle (foot just hovering on the accelerator pedal) at virtually any speed, if I then press hard on the pedal to accelerate quickly there is a brief thump or jerk as the car increases speed. It's exactly the sort of issue I would expect in a car where the universal joints or the engine/gearbox mounts are worn or loose. My very knowledgeable long-term Subaru mechanic today checked all the mounts, UJ's and suspension bushes and found them all serviceable and in good condition. He suggested it might be the Idle Speed Sensor but wasn't able to remove it to check as it was screwed on with Philips head screws extra tight. He didn't want to cause more problems by forcing it when there was no guarantee it is the cause of the problem.

Has anyone had this issue?