I'm having a problem similar to this (actually, seemingly identical) except that I am only trying to display 200 posts. More posts exist, but WordPress seems to fail to get any more than about 150. This is after I increased PHP's memory limit to 256mb. Previously, it could only get about 60 pages using 32mb of memory. This seems like a lot of memory to get relatively few posts. Could there be another factor that is limiting the WP_Query results?