1. Check vcc. Verify with dvm that it is 5V and double-check the datasheet to make sure vcc and ground are applied to the correct pins.
2. Verify with ohm-meter that pic, modulator and oscillator all share a common ground.
3. Make sure all digital inputs are either driven at all times or pulled up (or down). In particular, if the modulator has a bidirectional pin that is connected to the pic, make sure it never floats; it may need a pullup resistor if there are times when neither the pic nor modulator are driving that net.
4. Verify the clock into the modulator with an oscilloscope. Look for proper amplitude and frequency, and verify that the voltage never goes below ground or above vcc.