Hmmmm I was going to advise to get the radiator and block flushed but it seems you have already done that! Has there been any change to the size of the radiator??? Were any hoses changed when it got flushed?? Also your fans??? check that they are working when idleing for a long time like when you said you were sitting in traffic, (don't look when stuck in traffic, bet the lights go green juz as you pop the hood )
Do the fan check in your driveway just start the car and see if the temp skyrockets least you won't get stuck away from home!