You can configure a pulse which fires at random intervals within the Stimulator Waveform Customization panel. Access the Stimulator from the Setup menu and click the Customize button.
In the Segment parameters for a pulse segment, type the function random into the End Delay value box. The random function will provide a value between 0 and 1. You can also include further arithmetic to change the randomization limits; for example, multiplying by 0.5 will randomize values between 0 and 0.5.