Video Clips go dark and grainy at start of each

I'm trying to create a "trailer", using a selection of short clips sewn together with cross-fade transitions between each clip. As each clip starts (both in preview and after exporting), it is dark and grainy for a few seconds before returning to full quality for the rest of the clip. Can somebody help me with this? It's not due to the cross fade as I tried removing these and it was still happening. 

Thanks