overheating when in traffic but not when driving could be number things including the head gasket. This problem is an indication of a blocked radiator. If your radiator was cleaned out correctly (not just back flushed with a hose) check the flow like ddgonzal said, your flow should be really good.
Did the people who cleaned it tell you if it was blocked? The reason I ask is often your car will overheat due to a problem and as a result of this, the head gasket will then blow. So you may have fixed your original problem but your car is still overheating due to your head gasket.
You need to diagnos the problem before you pull the head off.
1) This is exspensive and you dont want to fix it if its not broken and
2) You dont want to overheat your fresh head and gasket.
A few things to check
Your fan is big enough
Your fan spins to correct way, it should draw the air through from front to back
Your condensor fins are not clogged with dirt so you have good air flow
Try to do a compression test and a TK test on your head gasket
Check for milky oil and check under oil cap.

You could always pay a mechanic to diagnos to problem and have a go at fixing it yourself. Good luck