new thermostat & water pump..still overheating, help!b

[fongo]

Memorial Day weekend blues for me!
On my 93 5.0, I just put a new water pump, new thermostat 180 deg, hoses, flushed out the Motorsport 3 core radiator radiator...
After turning the car on for a about 15 min & running the A/C, my car temp gauges starts climbing towards the 270C limit. I checked everything: no leaks, new parts, fresh coolant etc. I did put on a new 95 amp alternator (PA perfmce) yet I dont anticipate any problems? The serpentine belt is tight to specs.
However, after I ride the car around the block a few times, the temp gauge does go down? Could it be a defective radiator? It does get hot though. Could it be that the radiator is not compatible? It is a little thicker than the stock and the fan shroud tends to be more closer to radiator than before. Need some tips.
Thanks! Hapypy Holidays!
fongo