Hey there, I'm assuming you've had the mouse for a while? After about 1.5 years of use my mighty mouse started doing the same.

It probably needs cleaning. I took mine apart to clean it (not recommended as it involves glue to put it back together) which seemed to work fine, but a lot of people have been using rubbing alcohol to get the muck out.

I think I recall someone saying holding the mouse upside down, using a little bit of rubbing alcohol while pressing down hard on the scroll wheel and rolling it around does the trick. Might need to repeat the process once or twice to get it back to its former glory though