I have the same problem on one of my sheets (using the default stylesheet). I started from scratch and re-wrote that sheet, still using the default stylesheet; and, the cursor is acting normally. So, it is not endemic to Mathematica, but must be in in that one sheet. I cannot find what's different about the sheet that is making it behave so.
By the way, this is on a Mac Pro running Mountain Lion, 10.7.5.