Red and Green Problem

Hi everyone,

I've suddenly started having a weird problem. Some clips play in the program window and render with either blank red or blank green filling all the frame.

It happens intermitently. Sometimes I can play the clip all the way through or render it without it happening, other times it happens right from the start.

I'm using the same settings I've been using for months with no problem.

Does anyone have any idea what the cause might be?

Thanks in advance