Just because the thermostat and water pump are new doesn't mean they are GOOD. I've seen thermostats installed upside down and because the people doing the work didn't know how to properly refill the radiator step by step. Instead, they partially refilled it and sent the owner on his way.
Still, it wouldn't hurt to have the radiator flushed especially if you haven't been changing the coolant regularly. I personally flush mine every other year and add new coolant.
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