asteroids screen problem (a dead zone)

I've been restoring an asteroids cabaret and it's almost there but a
small intermittent glitch has appeared in the video.

The game plays fine except for when there 's very little on the screen
like the start of the game where it says 'player 1' or when you're down
to the last asteroids. A vertical zone sometimes appears in the middle
of the screen (running top to bottom and maybe about 1cm wide) in which
everything disappears.

There's no distortion in the vectors even as they drift into the dead
zone. It's almost like playing the game with a piece of black electrical
tape stuck to the middle of the screen.

Anyone got any ideas?

---
James G Watt
http://www.kinetic-arts.co.uk
Part of the Video Arcade Game Collectors WebRing.
You are in a maze of twisty little passages, all alike.
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