It sounds like you are overexpanding to me. Unless you are a very good player, it is pretty difficult to come back after your units have gone on strike. You end up getting too far behind.
I would make about 6 or 7 good cities and that should put you around 20-30%. You need to whip courthouses, libraries, markets and forges. You can then recover by specializing your cities. You can build a gold city, science cities, production cites, etc..
After you recover, you can squeeze in a few more cities. You just have to find the right balance. It helps to block off your neighbor or neighbors and have some land to expand to later on.
This has been working for me on Prince, but I don't know about the higher levels. I will be on Prince for a little while longer.