What is causing my car to run poorly and get bad gas mileage?

It is a 97 eclipse spyder with the 4g64 engine (non turbo). It has around 145k miles on it. It idled very rough when in neutral until I turned the idle speed up. I replaced the air intake with an aftermarket one. The filter is clean so I doubt the intake is the cause. I also replaced the mass air flow sensor and spark plugs which didn't help any. These cars are estimated at 23 city, 31 hwy. I average around 18 mpg, and with gas prices as high as they are, I need to fix something. When cold and accelerating, it feels like its dragging a train, then all the sudden it will accelerate quickly and smoothly (only does this when cold). I'm not sure if its the o2 sensor, a vacuum leak, or maybe a computer problem.