Hi Good day, I’ve had a few issues with sync with my NTSC SNES on the OSSC also my cable was CSYNC and I also had one with Sync on Composite in some cases that gave me some jitter on the screen and even some audio and Sync issues, What I could recomend you is getting a “Sync on luma” cable I had no issues with my OSSC or Sync after I change my cable.
If you have the Skills you can change a “Sync on Composite” cable to a “Sync on luma” easily.
But You can also just buy the cable online as far as I know this helped me a lot and I havent had any issues since then.
PD. Make shure your cables are shielded becasue sometimes that can affect the video and audio quality of the image/sound.
Also check if the cable is loose or somethin inside, That also happend me once with a saturn cable.