For a while i've been noticing that above 50mph, when trying to coast, I get an odd and annoying problem. When I take my foot off the accelerator, the car jerks me forward, and when I press back down, it jerks me back - and i'm not talking about an abrupt and significant change in throttle, i lightly press the pedal and the car jerks & i'm pressed back into the seat, and when I remove my foot lightly the car jerks and i'm pressed forwards. It's frustrating because when i'm trying to coast at 50, i either have to put my foot on the gas and go faster than 50, or remove my foot and go slower due to engine braking, or deal with the annoying lurching back and forth to stay where I want to be.

I thought that the throttle position sensor may have the "closed" position set too far from the actual closed throttle position, so that by the time the TPS gives the signal to the injectors to begin firing again, the throttle is already a bit open so that I get an abrupt jolt from the greater air mass giving more power. Though i'm not too sure how that would explain the inverse happening when I lift off the pedal.

I noticed something yesterday which made me doubt it's entirely throttle related - the issue only occurs at/above EXACTLY 50mph, and doesn't occur at all below that - not at 49mph or lower. So that kind of throws my theories out the window.

Any ideas?