Did you find a new area to hunt? Could be mineralization is causing it to read wrong. Also, if you've been hunting the same area for awhile, you may be finding deeper and deeper coins, and most detectors have a tough time getting the ID dead-on at some depths. On the other hand, the detector may just be broken.
Don't I know it! I have my own little slice of heaven in town here that constantly gives up goodies, no matter how often I go, but for some reason wheats at depths of about 5-6 inches show up as a clad cent on my machine. Nowhere else, just at this park. Since I don't dig clad pennies, I always check the depth of any clad cent signal, just in case.