2000 Mazda 323 engine problem

You might try a can or 2 of good fuel system cleaner. BG44K is best. You have to go to a car dealership to get it, specially at the parts department, and it costs around $20 a bottle. Run 2 bottles, each on a full tank of gas, then run another tank of just gas before evaluating performance changes. It may actually make the engine run rougher at first while it removes deposits. This will all settle in the oil pan so it is a good idea to change the oil after the third or un-treated tank of gas.
The Mass Air Flow Sensor or MAF may be dirty or need to be replaced or calibrated. You might want to check fuel pressure to see if it in in spec as the fuel pressure regulator may be bad.

It could be the camshaft position sensor or maybe the timing belt has jumped a tooth.