For a few weeks we have been having the occasional flickering screen which has been sorted by restarting EW.

This Sunday our normal theme of white letters on black background flickered even after several restarts but our streaming theme of a small text area with a blue background worked perfectly.

Has anyone else had a problem with the output flickering so much as to be unreadable and what sorted it?

That is why it's important to mention EVERY point in the chain in a support request. 

I think I have solved it.

We feed our output to a Roland video mixer and it is this that is misbehaving.

Thank Donal.

It is getting odder and odder. A new blank theme and an imported one from another computer both flickered but a theme where the backgorund graduated from indigo to black behaved itself.

Also a picture with a substantial amount of black behaved itself.

Quite possibly. Different colors react differently. It could also be the display itself. Possibly EW Support itself would have some other ideas to look into.

Would that explain why it is fine with one theme and not with another?

I've only encountered that with an under powered graphics card or a bad cable, and mainly the cable.  Especially over an older long USB cable.