At the risk of getting back to the thread, isn't the fact that the incident was so long ago part of the issue? We still have these venerable old birds on our books.
Anyway, my understanding is that, by the time the nav had done his sums, the star shot told you where you had been half an hour ago. Plenty of time to stray off track in the meantime.
Is the same airframe still in service? That might shed light on the scale of the age problem.