If it's not slop in the drivetrain and your injectors are fine then I would suspect there is either still a problem with your tune or there is a fairly large vacuum leak somewhere (or smaller leak on the vacuum line to your MAP sensor).

 

Tune seems to be great, in saying that this issue has happened for many years. Car has been tuned many times since then going from twin turbo to Single turbo. The engine was rebuilt not long ago with all vac lines replaced. I can't imagine it would be a vac leak?

 

Check for a sticky or loose throttle plate?
Idle speed control valve malfunction?

 

Not sure how you can assess that the tune is great when you have severe bucking for years. Even if the tune was great nobody would really know. Who are your tuners? Are they always starting with what's already on your ECU and just tweaking it? Are they starting with their own base maps?

Vacuum leak can be anything, not necessarily something obvious like a manifold gasket. But any air getting into your intake that the tune does not account for or air leak that causes incorrect MAP reading, that's a problem. Things like a stuck IAC valve, cracked vacuum line, etc.

 

My FD has been tuned multiple times over the years by a number of different tuners. My current tuner has quite the reputation in Aus and has felt the best right now minus this issue of course. The entire tune is custom, has his own base map then adjusts accordingly.

When I let the car decelerate down, especially in 2nd, the car heavily jerks back and forth and alot of it can be felt through the transmission which made me think its electrical, its even worse when the car is cold. Not to mention I have an auto harness which I just discovered. Even when I let go of the throttle at lowish rpm, you can feel my gear stick jerk < again, thinking its electrical.

 

I tried to record what it does..

 

Heres a better video

 

How do I test to see if the dashpot is working correctly?