Philips dvdr3440h picture problem

I just got a Philips DVDR3440H recorder with 80GB hard disk.

I have connected my digital set top box to the Philips using a scart lead to the EXT2 input.

When I view or record a program from the EXT2 source I sometimes notice a series of diagonal stripes going across the picture. This is more noticeable on high contrast pictures with lots of solid colour (eg a blue sky or a solid area of single colour on cartoons etc).

Does anyone know what is causing this. If the Phillips is switched off and I watch the set top box the picture is fine.