Computer simulations of FEL lasing differ in the degree to which they approximate real experiments. One of the FEL codes used extensively at Los Alamos takes account of the features of each electron micropulse and follows the growth and saturation of the optical micropulse. With no additional adjustments, this code displays the development of sidebands and demonstrates their control when optical filters of various kinds are used. Other codes that do not include a description of the micropulse do not automatically display sidebands but need to have artificial noise of some kind added. This is not unexpected because sidebands are generated by an FEL instability; instabilities, in general, need some kind of initiating disturbance. In this paper we have (1)|identified the disturbance that triggers the instability in the pulse code,(2)|discovered a practical way to suppress the instability without using filters, (3)|compared these results with experiments, and (4)|discussed these findings.