How is the filter pressure doing? Backwash when it goes 8-10 psi over clean filter pressure. You might try some skimmer socks and also adding a little DE to your sand filter. Info in this thread:

http://www.poolforum.com/pf2/showthread.php?t=3742

Sounds like you may be on the home stretch! Keep hammering it with bleach. You are doing a good job.