around 1000 A.D. I start losing money and happiness. I've tried changing gov type, lowering the tax.
If you're lowering the tax, then that's why you're losing money. Without knowing anything else about how you play the game, these symptoms sound like a deficiency in economic improvements. By 1000 AD, both the Marketplace and the Bank should pay for themselves and produce surplus money in every city which has them both. And both of these improvements contribute to the happiness of your people.
Instead of dealing with a crisis in 1000 AD, plan to not have a crisis. Some easy techniques:
1. Don't muck about with the rates very much. You have 3 rates to set: Luxuries, Gold and Science. Before you get Religion, keep your Gold and Science fairly matched. If you don't get the Hanging Gardens or Oracle before a rival civ builds those, then you may have to nudge your Luxuries up to 10%. Otherwise, keep Lux to 00-10, Gold at 40-60 and Sci at 40-60%. Early in the game, balance Gold and Sci. You need both later on.
2. In your initial exploration of the lands around you, your second focus (after enemy civs) should be on finding a place to build a city which will be the future home of the Colossus and the Copernicus Observatory. Look for gold in the mountains, gems in the jungles, or a citysite which can eat 3 fishies. When you identify that site, develop the area and found that city as soon as possible. It has a long way to go, so start it early. Once it's up and running in the midgame, all your troubles are over.
3. Pay attention to Marketplaces and Banks. In the early game, my fresh cities will do this build order: Settlers -> Granary -> Temple -> Settlers. The fifth build is either a Barracks or a Marketplace, depending on the terrain of the city and the current threat level. Once graduated out of Despotism, a good build order is: Settlers -> Temple -> Marketplace -> Granary -> Settlers. See how that works? Early on you need fast population to support a military to take possession of your homeland. Later, your Gold and Science are a surer path to world domination. In either case, the Marketplace comes as a very early build. As soon as a city is earning 6 gp per turn, they need a Bank.
4. Mind your growth. Before having a Cathedral, a city should not grow faster than 1 or 2 wheatstalks per turn (aka single-growth or double-growth). If a city is at 6, redirect the people to build a Settler to knock the population back down to 5. One garrisoned unit and a Temple will keep a city quiet up to 6 population if you have the Oracle or Hanging Gardens. Don't outpace your capacity. Without Religion, 6 is your max, and 5 is more comfortable. When a city at 6 can't make a Settler, adjust the worker squares to put that 'burg at zero-growth.
5. Once you get Religion, feel free to adjust the rates however you need. With a Temple and Cathedral, no garrisoned city will go sour on you. The extra population a Cathedral enables (beyond 280,000) will earn you plenty of money, and luxuries, and knowledge. Use the burst of knowledge to get to Women's Suffrage, switch to a Democracy, and you will never have to worry about lack of money or unhappy people again.
) and I keep having this problem, around 1000 A.D. I start losing money and happiness. I've tried changing gov type, lowering the tax. Not should that I'm doing wrong here.
