Could be a memory issue. Hair and cloth sims are memory-intensive, and Poser's famous memory leak takes its toll after awhile. It's a problem in P5, too, but not as bad as in P6.

If the sim's not running right, I exit Poser and get back in again, and it's usually fixed.

With P6, you should save often, under different filenames, because it tends to nuke the whole file when it has memory problems.