UVC black scrolling lines ?

[Trevor spencer]

Just hooked up my UVC and it works fine in 15khz mode but on a white screen ive noticed some faint dark scrolling lines that scroll really fast up or down and then slow down and disappear then start again? Ive tried an extra ground wire from the jamma harness to the body of my cab but that didnt work

I have a sega aero cab btw

Any ideas
Cheers