Sound like a fan problem. anytime the car starts to overheat at a stop light but cools back down as soon as air is flowing threw the radiator is a dead indication the cooling fan(larger fan) is not kicking on when its supposed too. if your hearing a fan it could possible be the A/C fan but that alone is not enough to keep temperatures in check.
start at the fan and work your way back. Colin did an excelent write up on how the system works this might be helpful in diagnosing it.
i had a similar problem with my old LS when i would use aftermarket thermostats. i notice the temp would go up and sit about 3/4 when going up hills under heavy loadm and then drop back down when im at cruising. thermostats can get stuck in ANY position. partially open, fully open etc. theres no rule of when or where it fails. also, make sure the fans turn on without the A/C on as well. it could possibly be a relay or even the fan switch not working. one thing to check as well is if the fans DO work properly at idle, grab the hose connected to the thermostat outlet and make sure the hose gets just as hot as the upper hose. if it does, you can assume the thermostat is opening