Pl let me know the solution for given below problem

While replying to my emails some times it works fine but most of the times my screen hangs I m not able to type anything

on it. It shows very thin (light) line on left side. When I create a new email it works fine even all other products like word & excel are also working fine.

I m using Mac OS X 10.9.3, Pl can you suggest me a solution for it.