My brush keeps changing thickness?

I loaded up Illustrator the begin work on a piece only to come to a halt when my brush tool is random changing its thickness with every stroke?

The image below shows all the lines and how the thickness changes with every stroke. They all seem to be completely random.

(I have pressure on but no where near to the sensitivity it's showing. No matter how light or soft I press they all come out random.)

Anyone have a solution to this?

Thanks.