Check for leaks , pressure test radiator to pressure on cap, gauge must hold pressure for several minutes, if no leaks release pressure, leave gauge in place, start and run engine , if pressure rises on gauge faulty gasket or cracked head, if ok the radiator core may be partly blocked . Reverse flush may cure this.

You have a GM common problem and that is a bad intake manifold gasket. GM change the gaskets to plastic and they crank and leak causing over heating. here is a picture of one i took from a 2005 pon. bonneville with 74,000 miles on it

GM trouble leaking gaskets.IN side or OUT side.I try on 99 grand am HEAD GASKET FIX for $30.00 work well,two years no leak.

Check your fan if yo replaced your thermostat see if your fan is on