1997 Mazda 626 Fuel problems

Ok my car has had the head shaved twice. Now the problem is that the car has a serious lack of power when it is low on fuel and it often hesitates when the tank is full. I have noticed that decrese in the gas miliage. Any suggestions other than the fuel filter. Plugs and air filter. The car runs good and strong in the higher gears but sometimes it really looses power in 1st and 2nd. The most problem come when the car is low on fue as in 1/4 of a tank. The car also idles irraticaly. There arent any lights on.