flicker on certain shots in iDVD

Hi,

does anyone know why some of my 5D footage pulses/flickers once on DVD?

It's fine on the ProRes (I shoot 5D and then convert to ProRes) but then certain shots, particularly close shots where there's an out of focus background, seem to kinda pulse and throb a bit, almost like the compression can't handle it.

Almost like a moire effect where u get lines too close together, going a bit strange, but not quite that.

Has anyone ever seen anything like that and have any idea what it could be? It's usually always where's there's a lot of out of focus background in the frame.

Like I say it's fine on the Quicktime itself, no flicker at all. And shutter speed whilst shooting is always 50.

Wondering if my iMac is on the way out maybe? Around 6 years old now.

Or maybe it's cos I only use iDVD and maybe should use Studio Pro for a better compression? Not sure if that would make any difference?

Any ideas would be MOST welcome!

Many thanks!

Mark

UK