I had a lot of trouble getting from level 3 to level 4. With save time set at 180 the crawl would crash before it had a chance to save. I finally set the time to 45 and it was able to get past to level 4. Then things were reasonably smooth except it would crash every 800 pages or so, but would continue from the last save. But when it got to 13714 (current save), I could not get it to stay alive long enough to make a new save. I tried 45s, 30s, 20s and 10s for save intervals but in each case it would crash within 200 pages and not have made a save file. Then I tried 180s. It ran for about 800 pages and then crashed (with no new save record).
Is there a threshold related to memory use? When I was trying to get from level 3 to level 4 things got rough when memory was about 15,300 Kb. After going to level 4 the memory dropped back to 12,000 Kb or so, but slowly built up to the 15,000 Kb level. That's where I hit a wall. Splat.