Bmw e30 chokes/shakes and jump in low rpm

I’m having a problem with my beloved car. An bmw e30 316i. So when I’m in any gear (except reverse - manual transmission) and the car is moving, if I lift the foot from the accelarator and let the rpm drop to idle (arround 1200 rpm) the car starts to shake and jump up and down quite fast. But if I press the clutch pedal, the car stops doing that. If the gear is in neutral the car never does this. I had a peugeot 206 and did the same, but not so violently like this e30.

Ps. Me and my dad worked in the car few days ago. We took the intake manifold to replace a fuel pipe and repair a vacuum leak. Before that the car didn’t shake the way it does now. What I am thinking is that the ecu now has a steady flow of fuel and air to the engine and once it reaches low rpm’s, the ecu cuts the fuel supply to the engine (resulting in a choke of it).

Sorry for this long description of my problem :)