First of all, there is no way your income can jump from 100 gold to 0 gold a turn and not have it be golden ages. Golden ages are huge, and I go from +50 gold to +150 gold, and back, due to Golden ages. Otherwise, it could be you completed something like 6 - 10 new units in those 5 turns, whose additional maintance cost could cost 100 gold. My advise: don't build units you don't need.
Second, city states are too powerful, if anything, not too weak. Maritine city states are must-buys, even if they are hostile. Your cities grow so fast, and for free, so you can focus on building more trading posts than farms.
Culture city states are great too, they can help you get many more soical policies, which are free bonuses.
Miliatary city states are situational. They tend to suck badly if you are more advanced than the rest of the other civs, as they give you outdated units. They are also bad early on, as the cost of their alliance exceeds the cost of just directly buying early units. However, on high difficulty levels, where the AI's often have out-teched you in military techs, and later in the game, you can for a cheap 500 gold get units that cost more gold, that you might not even be able to build yet (because you have been focuses on slingshotting to other useful tech).