I have complained about this a couple of times, in at least one thread - I thought on this forum, but I can't seem to locate it in a hurry now.  In short, since a recent update of PMail, probably the one to 4.41 or to 4.50 beta, some change was made to the editor which produced symptoms which sound rather like yours.  In my case, what I noticed - and still do moderately often in version 4.51 - is this.  You start on a message, then need to go back and modify what you have already written, e.g. by inserting some new material in an earlier sentence.  In those circumstances, you often find that the insertion point has jumped away from where you were sure you had placed it.  I'm a fast typist, but not a touch typist, so I type away, then find I have garbled the message by inserting my extra material in the middle of a word.  It's a shock and an annoyance every time.  The actual insertion point is usually close to where it should be, I think always on the same line, but it has moved, without warning and for no discernible reason.

best
David

I type pretty fast, ~70wpm.  There are times when I'm blasting away and the cursor will randomly jump to another line (often within other text) for no apparent reason.  Obviously I've considered the possibility I'm hitting a stray key and making it happen, but I can't imagine what key that might be that could cause that (unless a combination of several alphanumeric keys can somehow create the action of an arrow key).

 Has anyone else noticed this?

Yes, I've seen that happen. Not very often, but occasionally.  I can't remember if it's happening with Pegasus or with my browser when posting to forums, or both. I first thought it was being caused by the touch pad on my laptop being turned on, but it kept doing it after I turned it off, so I've been assuming that I must just have a loose connection in the laptop since it's had fairly rough treatment, but seeing your symptoms make me wonder if it's something else. I don't type very fast in general, but there are a few keystroke combinations that I tend to do quickly, so perhaps some double keystroke is being interpreted as a move cursor or something. 

 Sorry I don't know the answer, but I'll be interested in seeing what other people say.