Engine is running hot, most at red lights but sometimes moving it will get a lil hotter then normal?

3.8 v6 in a grand prix with a supercharger. radiator replaced a few months ago. i have flushed out the dex cool at least 6 times but it still goos up the radiator cap and the fluid looks nasty after a few weeks. i am mostly adding water and thinking i should add anti freeze. i also wonder if this goo on the cap is hurting it. she will run cold as can be driving then go up to about 220 sitting but its still not close to the red. the fans are both running full speed at idle. im hoping its not a water pump.