I have had the same problem. It seems that the only fix is staying away from custom plops, or balancing any super tall plops from downloads with a lot of in game High Tech Industry. And I mean a lot. You basically have to have what the game considers its own jobs out weighing anything you've added to it. Otherwise your income falls off a cliff as you plop and build your cities. I guess the average income is something the file creators here have not looked into as they are designing for overall demand and astetics more so that desigining for long term game affects. Hope this helps.