I've always had this problem even after replacing the clutch.

Say in 1st, 2nd or even 3rd gear at lower RPMs, if you were coasting and released the gas, and then stepped on the gas even smoothly and lightly, the car kangaroos (jerks back and forth).

This can still be observed a little bit in 2nd gear even at 2500 RPM.

Often you can hear a sound that moves to the rhythm of jerking which is whining sound coming from the clutch or transmission.

Plugs have been recently changed and it made a slight improvement.
I had new complete clutch kit installed by Subaru including a machined flywheel and the problem actually feels worse.

Another time you can feel this is if you shift into a gear without giving it much gas, everything is smooth but when giving it gas the car jerks back and forth.

What could be causing this?

The only other fault I guess may be an issue is that my car stalls upon startup on colder days (never when it is hot) and I believe this is my IACV (also when colder the revs fluctuate by 300-400 RPM before the car warms up, as if I punched the throttle). I know my temp sensor is good since I replaced that.

Any help would be appreciated.

Thanks guys