VECTOR: Star Wars - Scale Test

Has anyone come across this problem before on a Star Wars AVG board - go 
into test mode and bring up the Scale Test (Linear). This should draw a 
smoothly shrinking box - however on my own AVG board though this doesn't 
happen, instead it draws a large box, then a few seconds later a much 
smaller box, then a few seconds later a much smaller box, etc. In other 
words the 'in between' boxes aren't being drawn.

I thought this would be a fault in the scale circuit, yet I can find 
nothing wrong there whatsoever or, for that matter, in the circuit which 
that feeds.

Any tips please anyone? What am I missing?


Thanks,
Phil