Tom,
Clearly something is amiss. My two theories are:

(1) Your CYA level isn't what you think it is (this seems fairly unlikely). How are you testing for it?

(2) Something is alive in there and is consuming your chlorine. If that's the case, shocking is the answer. Based on Ben's Best Guess Table (http://www.poolforum.com/pf2/showthread.php?t=365) you'll need to bring your free chlorine to 15 ppm until you can get it to hold some free chlorine overnight. It will take about 9 gallons of 6% bleach to get you to 15 ppm initially (that's a big pool). After that, test frequently and keep adding enough to bring it up to 15 ppm.

Best of luck.