What I would do after making sure that the motor has been thoroughly flushed, is as follows (one step after the other, only if the problem continues):
a. Clean the radiator passages as noted by another poster above.
b. Have a technician (or yourself if you know how), check the radiator cap to make sure that it's holding pressure. Also, to make sure that the pump is working properly. A pump not working properly can result on overheating conditions.
c. If the right thermostat was installed, then I would double-check the antifreeze/water ratio instead of adding chemicals to the antifreeze. In fact, I would completely drain the radiator and motor and start from the beginning with a new mixture of antifreeze/distilled water. Sometimes having too much antifreeze or too little water can cause a motor to run warmer than usual. If you have used pre-mixed antifreeze from the store, this one usually is mixed at an antifreeze/water ratio of 50/50. In this case, try adding enough distilled water to make the ratio something like 40 antifreeze/60 water and see what happens. Just stay close to the proper mix ratio for cold, temperate, or whatever region you live at.
d. If everything fails, then pay attention to the wind when you drive. Is it windy outside, and are you driving against the wind direction? If you are driving against the wind, driving uphill, or just towing, the results are the same: higher engine temperature.