Using IE8, when I make long posts (long enough to need to use the edit window's scroll bar), the text jumps around unreliably and refuses to stay put, as though I was constantly dragging the scrol bar back and forth.

Since noticing the issue, I've not had a chance to try any other browser to see if it carries over.
Even if another browser solves the issue, some users may not have other browsers available.

Thank you for your report. I've checked at the phpBB Support Forums and this seems to be a common problem.

I'll investigate and implement a fix once I'm back from Reading.

EDIT: worth noting that this is a known IE8 error, and not a phpBB bug. To fix it it looks like we have to take advantage of another known IE8 error. Nice work MS, nice work.