PMDG have done such an amazing job modelling this aircraft, i'm gonna venture to guess this is normal, real aircraft behavior, but when i'm on descent and in VNAV mode, the planes seems to slowly pitch up and down. It is very small pitching and not jerking or anything like that, its only maybe a degree or two up and down. Is this normal? Sarah Connors

Yes. It's either trying to stay on its path or its speed.

i also had the expeirence like that. i swa the column go forward and aftward don't know why?

Completely normal as Spin737 mentioned it is staying on it path, its very slight movements but when you are looking out the front it is more visible due to fixating on the horizon. If you were in the back of the plane you wouldn't really notice the movement. Next time you are at the TOD take a look at one of the wing views the transition is almost invisible unless you are really looking for it!