Just started a new city, no salvation for my old one at the moment...
For now all looks fine, demand for all zones and densities, balance between workers and workplaces, but only appr. 20k ppl yet. I'll build it up and see what happens.
Somehow it seems that things run smooth until the city reaches some bigger size, then it's getting worse.
Maybe the simulation is doing some miscalculations, only few, perhaps only minor ones... as long as population is relatively small this has no effect (at least not noticeable), or the effect is compensated, but with constant growth it cumulates and at some point all switches over and breaks down.
Or it has to do with the patches, the first city was started on release day, and the patches broke something down irreparable.
3rd theory is the freaking high land value (up to average !!! 120k$ per square in my broken city) preventing anyone from outside moving in.