OK, 2 major discoveries....

1 - It's not the pinball that is causing the main problem, but it is part of it

2 - The 2 sheets of steel seems to work, but I don't know for how long of course.

The pinball is causing the problem to the left of the screen (pinball is on the left) but something else is causing the bigger problem at the top right. If I use the 2 sheets of PC case Steel the problem goes away, so the monitor itself is fine. I have removed the surround sound speakers to the right of the arcade cab but it's still there, in fact I have now removed all electrical items from near the cab and it's still there. I can't think of what else would be causing the problem.

For the time being it doesn't seem to be causing a major problem for the monitor, but now I have noticed it I find it hard to look at the screen without getting annoyed.

And of course there is the problem of mounting 2 sheets of steel....

Cheers

Cris.