X-Plane 950 beta 1 should fix the stuttering present in 940. The bug was: the more you flew in one area, the worse the stutters would get - they'd clear out if you kept flying forward enough to scroll DSF tiles normally. Anyway, the bug was a book-keeping error based on the new "paged" airports from 940.* Fixed now - thanks to the users who got me a really clean test case to see this.
(Hint for users: if you can duplicate a scenery-related bug by saving a replay, or even if the bug can be reproduced in replay, that's really, really useful to know! It makes the bug much more repeatable, since what scenery is loaded varies a lot with flight path.)
* Before 940 we would load the entire apt.dat file into memory on startup. With Robin receiving new highly detailed airport layouts every day, this was starting to use more and more memory. 940 and 950 will load a litttle bit of information about every airport and take note of the file that contains the layout - then when the sim needs a layout, it goes back to disk to pull up just the one airport in question, saving memory. We "page" the airports into memory only as needed.