I can do most any beginner exercise and i can do it fairly fast. However sometimes if im switching strings, i tend to strike the wrong string. Is there any exercise that focus on acuracy of the picking hand and combines speed, even if its eventually?

--4-3-2-1-------------- <- first
----------4-3-2-1------ <- might miss this one
------------------4-3-2-1-- <- then strike this one by mistake
---------------etc----
-------------------
-------------------