I don't know if this is a bug in my browser or what, but when I am composing a longer post in the text box of a COW discussion page, it acts as if some ghost is remotely scrolling the copy up and down randomly to read over my shoulder, right as I am editing, typing, or inserting characters into a word or sentence. I will have scrolled down several paragraphs to the bottom, say, and as I'm placing my cursor to fix a misspelled "the", the darn thing suddenly rolls up to the middle of the unfinished post!
This seems to happen in both directions: if I'm correcting something on the first sentence of the post, the thing will run away on me and dive to near the middle of the post, completely un-requested by me. It's sometimes very frustrating and has cause me to take twice as long to write something, as it normally should.
Again, I don't know if this is a bug caused by my browser, or something other people see when they make longer, multi-paragraph postings. Thought I should mention it here though.
Microsoft internet explorer on PC. I use another browser at home, am trying to remember if it happens there as well. I don't clearly recall at the moment, will post an update if it happens there as well.
This happens before any font tricks or anything, just typing in the box. When I type enough that the box requires a scroll bar, the effect starts to happen, and gets worse the larger the post gets.
Unfortunately that bug doesn't seem to be the same in ie9 or was maybe was fixed as I can't recreate it using that browser. I no longer have ie8. Based on that thread the ie8 bug didn't seem to be something feasible to work around.
Using the text manipulation tools does seem to jump the cursor in different ways in different browsers and I'm going to look at that now. The issue you're experiencing sounds different then this though since you say it is happening when typing or simply editing text.