Spartan 3AN FPGA DCM
You don't have CLKFB connected - I'm surprised you get anything!

From the Spartan 3 userguide, UG331


  CLKFB
  Clock feedback input to DCM. The feedback input is required unless the
  Digital Frequency Synthesis outputs, CLKFX or CLKFX180, are used
  stand-alone. The source of the CLKFB input must be the CLK0 or CLK2X
  output from the DCM and the CLK_FEEDBACK must be set to 1X or 2X
  accordingly. The feedback point ideally includes the delay added by
  the clock distribution network, either internally or externally.