Sorting by last activity is quite useful, but most of the time i try to avoid it because its too annoying. The property updates each second and with each refresh the sorting is different. The view is changing fast and this makes it hard to read and even harder to select a desired torrent.
I suggest a simple fix: round "last activity" value to lets say 5 sec marks (15:15.25, 15:15.30...), so all active torrents get the same value even if they had activity at sec 27 or sec 27.876. And then do a secondary sorting by another column, for example "created" or another 2nd last sorted column. The result would be that active torrents keep their position more stable for longer period of time.