shaky asteroids video

Hi list
I have been trying to fix my asteroids cocktail that has shaky video for years on and off....today I got closer to the solution.
The problem is worse during times when text is shown on the screen, I tried different monitors, different game pcb, different AR baords, Big blue etc...still the same. There was also a problem with humming on the sound...I thought the two were connected.
The sound problem turned out to be a bad fuse F5 on the power brick, only half the AC was getting to the game PCB and this must have had an effect on the 22v that feeds into the audio section, replaced the fuse and the sound was normal.
The wobble in the video is from the transformer being sat under the monitor in the cocktail layout, I pulled the monitor out to take some  voltages and powered it up on the floor...picture was steady as a rock!
I looked at some pictures of other asteroids cocktail power bricks and they seem to have an earth wire that goes into the inners of the transformer (internal shield maybe), mine doesn't have that wire. For now I have fitted a metal plate to the bottom on the monitor which helps a little but doesn't cure it 100%.
 
Has anyone ever come across this before?
 
Thanks
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Received on Thu Mar 11 09:59:59 2010