Here's the list off the top of my head:
Transition her to something other than sweet feed. Molasses will make some goat's milk nasty.
Deworm her thoroughly. Copper bolus. Get a good LOOSE high-copper mineral and put it out free choice.
If you think there might be sub-clinical mastitis, then send a sample in. Instructions are in the sticky at the top of the goat page.
Taste her milk right after milking and then later after chilling to see if it's bad from the beginning or after storage.
Don't shake milk in the jars, as it causes a structural breakdown that can cause goatiness.
What kind of bad flavor are you tasting?
Send her back, get your money back, and taste any goat's milk before buying her.