The AP of the A321-200 is horribly unstable, and because of this i cannot do a proper flight since it suddenly goes up even though i decreased the Vertical Speed to descend but it suddenly climbs.
Welcome to the Community!
The A321’s physics in IF are a bit weird - and it means it’s very susceptible to sudden changes, flying even slightly too slow, not being trimmed properly, etc.
Hopefully the A321neo will be better. 
Yeah, that is pretty well known, we are all hoping that the A21N will have better physics
If it’s built from scratch, then it’ll be ok.