I have a 98 Buick regal I changed the
thermostat and water pump and it is still
overheating what else could be causing this
might do a flow test with a small lazer thermometer~ blockage? perhaps the BLOCK was calcified with tap water at some point in history~ might be blockage or collapsed Hoses too, seen that most likely the radiator has bad flow, flow test that one possibly clean the block in an environmentally sound fashion~ today radiators are cheaper, check out a new one if yours seems all done with life~
If it's ever been filled with plain tap water several times or for an extended period sometime in it's history there could be corrosion issues. When tap water flows through a cast iron engine block and an aluminum head electrolysis occurs and is usually very rapidly corrosive to the cooling system components. The metals basically disintegrate and it will cause major expensive problems in a short amount of time. So always mix the right kind of antifreeze with distilled water anytime you're low on coolant.That probably doesn't help answer your question at all but it's not well known to people and I feel it's highly useful info. Jim's answer is most likely your best bet, I'd start with that. Coolant systems are great at trapping air pockets.