Battlezone - intermittent graphics glitch

I'm working on a Battlezone board set. It has an intermittent graphics
glitch - lasts anywhere from a fraction of a second to a few seconds in
duration at random intervals. When it goes "wonky", the vector
endpoints in text spread apart and the lines of text angle upwards and
become distorted.

Photos here:
https://picasaweb.google.com/108339843851140624101/BZIssue#

Before I start digging into it, any suggestions on likely areas to check
first?

I've already replaced all the electrolytic caps, the adjustment pots,
the header pins for the interboard cable, and any obviously poor
condition sockets. No faults come up in self test mode.

Thanks,

Alan

-- 
Total Impulse - Just Having Fun!
Alan Estenson, totalimpulse@comcast.net
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