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 pseudo randomly provide a value between 0 and 1. You can also include further arithmetic operators to change the randomization limits; for example, multiplying by 0.5 will randomize values between 0 and 0.5.
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