since upgrading from Lion to Mountain Lion I am also experiencing strange text issues to the point that my 27" i5 iMac is unusable for any application requiring text input. Because of this problem I am doing most of my work these days on my iPad.
Basically what happens is that I will be typing text when suddenly the cursor will leap to another part of the preceding text and insert itself. If I don't catch this immediately any typing I do overwrites the text after the new re-insertion point. Other times the text will suddenly start scrolling or a section will select all by itself and then delete. This is happening with pretty much every application that has text capabiity.
My iMac has 12 GB of RAM. So that is not the problem. And I seem to be clean from malware. The problem started as soon as I upgraded to Mountain Lion. My 27" iMac which is my main computer is now for all intents and purposes useless. Every day I pray for OS updates. But so far nothing.