Make sure the crystal your using meets exactly the specs required by Dallas (or Maxim).
From the datasheet for the DS1337 (which uses a different xtal as the DS1307):
The internal oscillator circuitry is designed for operation with a crystal having a specified load capacitance (CL) of 6pF. For more information about crystal selection and crystal layout considerations, refer to Application Note 58: Crystal Considerations with Dallas Real-Time Clocks. An external 32.768kHz oscillator can also drive the DS1337. In this configuration, the X1 pin is connected to the external oscillator signal and the X2 pin is floated.
Corrected, but I just wanted to point out the importance of having a proper xtal.