should i buy a new snowboard?

The instability will be a mixture of both you being a beginner and the board, which is purely for freestyle riders.

I would suggest a Bataleon board; it has a triple base which is good for carving in both groomed and powdered snow. It will also give you better balance on the slopes which hopefully will make it more stable - but balance does come with practice, the better you get, the more you will connect with the board.

http://www.bataleon.com/boards/ - they're pricey but if you're gonna use it a lot, it's worth it for sure! :)

libtech banana experimental?

LibTech doesn't make a board by that title. Is it a Skate Banana?

The Skate Banana is a more freestyle focused board. So, at speed it could be a little squirrely. It's also a reverse camber/rocker which is generally the loosest feeling of all camber profiles. It's definitely not a board designed for bombing down the mountain. So it can be both your inexperience and the board itself. I would practice the basics and take some lessons before investing in something else. I would put more weight on the fact that you're a beginner than the board.

You can't group all boards labeled 'freeride' together. There's a ton of variation in that category, just like all the other categories. If you want to judge whether a board will work for your application the basic things to look for are the flex, camber profile, and the shape. Other things play a role, but it can be confusing.

I don't have a suggestion as of yet because I'm still not sure exactly what you like to do on the mountain... other than occasionally you like to go fast. I also doubt the board is your limiting factor. Invest money in lessons.