Thanks for that Oshyan, the fixes you recommend obviously will help, but I'll have to wait a while for any upgrade to my system.
Interestingly, I returned to this scene to add a few extra objects i.e. a frog on a lillypad for example and found that if I positioned the frog on a lillypad and limited the crop area to just the lillypad and a little extra around it, then tiles within this small area would not render properly at all. If I increased the area to say half the width of the image then with no other changes, it rendered fully. This was reproducible.
Going back to the original tile problems I had, I noticed that it was always the same areas (tiles) that would not render fully until changes to populations etc.were made. Again these were reproducible for a given set of variables.