Space invaders monitor mostly works sometimes dim sometimes black

[Yohan]

I'm restoring a space invaders cab.  When I got it, it didn't do anything and now I can almost get a game in.  The problem that I'm having is that the monitor (it's the original) starts off great.  Then after a minute or two it will go dim then bright and back and forth a bit.  Then the whole screen will go black but the game still plays and then it will come back.  It will do this a few more times and then I shut it off.

I installed a cap kit and the image is more crisp but the problems still exist.  Any advise?

[RejectedManiac]

A tube that old I would hook it up to a tube tester. There may be a short in the tube.