Dreamcast video/no video

I have a Dreamcast motherboard which sometimes displays video and sometimes doesn't. The problem is present with composite and RGB SCART, because of this I am thinking composite/sync issue as it works fine with my VGA cable (which uses separate H and V sync).
Audio is present at all times.
I thought it might be an issue with the DAC chip so I swapped this for another but it gave no change.
When the video is displayed it works perfectly and I can play a game with no problems.

 

Have you tried a different monitor/tv?

 

I swapped over the GD-ROM unit from another console and it is working now. I put the original unit into the console that I took the GD-ROM unit from and that is also working.
I connected everything together correctly with the original configuration, took it apart again to test the board etc. so I don't think it was an issue of them being incorrectly connected.
It seems like they just didn't get along somehow.