1995 Chevy 454 P30 overheating

I would not think the waterpump would be the issue as the first thing to usually give on a waterpump would be the seals or bearings and it would leak. You say you don't believe it is the thermostat since it works at speed but maybe it only opens up when the engine is hitting a much higher temp only achieved under load and not reached when idling. You should be able to see when the thermostat opens while idling by watching the temperature gauge. When it opens (about 195 degrees I beleive) the gauge shuld lower a few degrees. If that is not happening the thermostat is not working or the coolant is being prevented from reaching the thermostat someway. I would have the radiator professionally flushed and replace the thermostat. Also check your primary cooling hoses from the radiator and make sure they are not collapsing.