This just started happening yesterday and it seems to be only Gmail doing it. When ever Gmail auto saves a new email that Im writing (which seems to happen when you pause typing for more than 2 seconds or so) the cursor is moved outside the body of the email so I have to stop and manually click it back to where it should be, as you can imagine this is making writing emails almost impossible.

Any ideas why this could be happening or how to fix it?

Try unplug your mouse.

If you're on a laptop, it could be your touchpad. Maybe over sensitive so your hand/wrist touches it as you're typing. There are sensitivity adjustments you can make, and also download any updated drivers for it. Otherwise, make sure your mouse (if you have one) isn't moving/bouncing on the desk as you type.

turns out it was just a problem in Chrome as FF was fine, just removed Chrome then re-installed it and all fixed