There seems to be a residual effect of a bad crack to the commodore 64 version of the city.  I had got back into playing it and was getting along when I noticed my characters would die after a few days to a week.  According to posts about this issue, I understand it to be a residual effect of the copy protection to the game.
 
After having much frustration with this issue, I started to play the atari version, since it did seem to suffer from this issue.
 
I recently stumbled upon a website that linked to a character editor for the c64.  I am not sure if this will help out, but at least you may be able to bring back a dead character.  I will have to see if it works.