98 pontiac firebird v6 3.8 liter, keeps overheating and i replace all cooling sytem parts.

When you said you replaced all the cooling system parts, you mean the raditor, the hoses, the water pump,the therostate, the raditor fan and it's relays and sensers. If you've done all of this and you correctly purged all the air out of the system then The engine can be making too much heat another way. The coolant temperture senser can say that the engine is cooler than it really is ( makes a rich mixture, to warm the engine) The therostate may be installed unside down or is open all the time or is missing. ( the therostate needs to make the car get up to 200degrees F or the raditior may not get hot enough to remove enough haet to cool the engine. ( hot is good too hot or too cold is bad). Let's say all the cooling system work is good and you did'nt mix more than one kind of antifrezze ( they jel up and not cool.) The exhaust can be stopped up ( causes low performance too) Take out the oxagen senser and connect a pressure gauge ( leave the snser plugged in and grounded) If the pressure goes any higher than 5 Lbs. reving it or anytime then you looking for a clog. A good system revved up and heald staedy should show a vacume. The timing can be wrong ( low performance too) The timing chain can skip time and the measureed compression will be lower that 150 LBS. With the chain skipped the ingtion timing will be slow and that will cause overheating all by it'self. The engine may need an oil change, Gas in the oil from short trips and running too cold sometime. Overheating will rurine the mototr oil and it being bad will make the engine heat. The engine turning hard ( turning the engine by hand you should feel 3 compression shrocks per turn and it should turn easy except as it pressureizes each cyclinder, ( they should all feel the same) the oil should look clear and black, not brown. If it's some other color, coolant may be getting in the oil, The exhaust should'nt smell sweet ( antifreeze leaking into the combustion ) with the car running and the cooling system full with the cap off it shouldn't bubble or steam come out ( it tells their is a leak in the engine to the coolant and it'll blow bubbles in the coolant and the hot exhaust will heat the coolant more than the raditor will remove. It certainly should'nt bubble at all started cold and full.