I'm not sure what you're asking. Do you have video of a video display or monitor that has a rolling sync bar in the shot? If so, the strobe effect isn't going to help.
Do you want to simulate a video monitor with a rolling sync bar?
Do you want to simulate interlacing (standard TV) where the picture is comprised of horizontal lines?
Sorry about the confusion. I was not sure what to call the "rolling sync bar", but that is what is showing up when I add the strobe EFX. And no...I do not want it to appear. I have set the strobe efx to show the under lying layer and I have also taken the Strobe efx off and set keyframes on the above layer to cut the opacity from 0 to 100 every 4 frames as to produce the same strobe efx. Both ways produce the unwanted rolling sync bar. I tried some bezier curve options on the keyframes, but no help...or at least I have not yet hit the correct combination.