Shock your pool up to about 15 and try and hold it there for a few days. Test 3x a day if you can and each time, add enough bleach to take it back up to shock level. You may just have something that you haven't killed yet and that is why it keeps coming back. If you'll look at the chart at the following link, you'll see that a FC of 6 is on the low side of the range that you should be keeping it at with a cya of 60. Hope this helps.
http://www.poolforum.com/pf2/showthread.php?t=365
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