Basically I just took delivery of a CLD-D925 and connecting to a display via the Composite RCA video and audio jacks just gives garbled, unstable video and a loud ground humming. Once I connect an s-video lead however it works just fine (really well actually), until I remove the s-video cable again and the rolling/hum returns. Is this some kind of signal grounding problem within the unit? Obviously I've raised the issue with the seller as it was described as tested and working fine with no issues.

Could also be a problem with the cable or the AVR or TV you are connecting to the player....

Yeah, sounds like you need a ground for your house and also different cables for the yellow video.
Also may want to try other discs to see if its the same issue.
Dirt on the connectors.

Lots of issues.
Good luck.

Turns out it was the cable after all...Everything is good now