Autopilot behaviour after departure

[AirSerbia]

Hello there guys!

Thanks for the great product you've created :-) 

I have an issue that is probably related to lack of my skill - so I need a bit of help on your side. Every time after departure when I engage YD, followed by AP aircraft starts to nosedive until it starts picking up speed again and climbing via IAS or VS.

I thought it might "chase" IAS that is preselected but the same happens with VS selection.

Anything I am doing wrong? Maybe the issue is the trim? Also I fail to find trim performance table, should there be any :-( Thanks a lot for your time.