Need advice on overheating 454
 

I know this is technically the wrong forum for this but it does have a 454 engine. The truck is a '99 c5500 which i believe is called a topkick. I am having problems with it almost overheating. If I start it up and let it idle it will reach operating temperature and stay put. If I am putting around town when I pull up to a stoplight it will climb maybe 10 degrees or so (according to the dash gauge) and then level off. If I take it on the freeway it will maintain the correct temperature but once I hit the exit ramp and slow down the gauge will creep upwards. If I hit a stoplight right after the ramp it will sometimes climb to 220 or 230, enough to make the check gauges light come on. If I rev the engine it will cool it down fairly quickly. I have changed the thermostat, mainly because they are cheap and easy to replace. I have verified that the gauge is pretty much correct via my scantool. I am leaning towards fan clutch but thought I would get some insight before I bite the bullet as it is about a $5-600 part. The fan will roar like they all do at first start and then quiet down. After it has reached operating temp I can stick a piece of cardboard in the fan and stop it. It takes a decent amount of force but I can stop it. I was thinking of removing the water pump just to have a look at it to see if it was corroded. Other than that about the only other thing I could think would be the radiator but since the truck only has 29,000 miles on it I would think it would be okay. Oh and no steam out the tailpipe and no milky oil or oily coolant. Any help is greatly appreciated.