Car overheating

My car overheats in traffic. I did a cooling system flush and clean and fill still overheats, the gan comes on, but does not stay on with ignition of, f also does not seem to be going in to high speed. This is a single fan system. Th thermostat seems to be working properly I can watch temperature go up to a point then drop as if the thermostat is opening. It does not overheat while idling or on the open road. I am stumped.