Hi. I don't know if this is the right answer....but I, too, have found that my scrolling would do as you describe when something has reset my security option to "high" after I've been on the web. When this happens I run Spysweeper and Ad-Aware programs (both free versions) to take out spyware that has gotten on to my computer. Sometimes it's been hard to change the security setting back to Med-high so that my scrolling "isn't possessed" as you describe. I often go to "Tools" and "Internet Options" and clear cookies, delete temp files and clear history that makes things easier, also. I am having problems typing sometimes ....like I type a sentence and it doesn't appear for several moments to a minute or so after I've already typed.....it takes it's time to appear on the screen. I haven't figured out if it is a memory problem where, maybe, something is running in the background and takes up memory to multitask......or if spyware or something may be causing it. Right now the Norton Systemworks Antivirus, Ad-Aware, and Spysweeper say my computer is clean.....but I still have some sentences, including writing this letter, that takes it's time appearing after I type it on the keyboard.
As for the "possessed" part of scrolling....I suggest to check how high the privacy and security settings are. I find that "High" won't let me scroll (when it somehow changes to that after being on the Internet)....and causes the screen to jump and spasm. I, also, find a spyware has gotten to it which I suspect causes the settings to change when that happens.