[Rick Parker]

Rick 427 Cobra above hit on a very good item. The OEM pumps in many instances used a cast iron impellor where many of the aftermarkets use a stamped steel which can cause all sorts of issues including cavitating.

Just another thing to throw in the mix: If you have electric gauges be sure the sending unit is the correct ohm rating for the gauge. Often they will function with the incorrect sending unit but the readings will be off considerably at one end of the scale. If you have a reasonable size radiator a 302 shouldn't heat up quickly at a light, (is the radiator clean inside??)