The mouse on my macbook was so aggravating that I almost got rid of the computer. My last bet was to check on your site and find an explanation for the erratic behavior.
Took about 3o seconds... all was needed was an adjustment on the J2 screw under the mouse.
So easy, all you need is the proper screwdriver. Not much to lose. I would try tightening the screw to the max and unscrew it slowly till the mouse works for basically any mechanical problem with the mouse...