Removing, cleaning the eprom and socket fixed the problem.
This was not one of the crappy TI sockets it is a Berg/Auget style dual side wipe.
First time I've seen a problem in one of those.
One of my standard troubleshooting techniques is to use a small screwdriver to lift all socketed ICs (not so much as to remove a pin, just to wipe the contact) and reseat them. Just a small twist of the 'driver at each end then reseat.
Dave
