I don't know about the plugin you use, but this may be because your buffer length setting is set too low. From the menu in FL, click options->audio settings. On the window that will appear you may see "Input/output" (I don't know what version of FL you are using) and beneath that, a dropdown which should list all drivers available. If you see something with ASIO in the list, it is best if you select that. If you select ASIO, a big button labelled "Show ASIO Panel" should appear on the window. Click it and find the control for buffer length in the new window that will appear. Increase it until you are satisfied with the quality of the audio. In case you don't have any ASIO driver installed or there is no improvement, you can still select your default driver "Primary Sound Driver" from the dropdown list and likewise increase the buffer length.
Hope this helps.