Having trouble using a Jack Plane to get consistent shavings

I have stanley plane like this one, i'm using it to shave off a few mm's of the top of oak peice about 3cm wide.



The problem seems that the blade only stays perfect straight with regards to the opening for a short while. After it will be crooked and cut more on one side of the wood. I always have to re adjust.



Is this normal or just an old worn plane?





thanks