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Well since there is stop leak or some shit in your radiator, there is a leak somewhere. 99% of the time its the radiator. You already did a flush, so you tell us. Did you see any leaks from the radiator?
If you didn't and you still boiled your coolant over then you could have several problems. Water pump, gasket failure around the water necks, head gasket....
A leaky radiator cap would not cause the overflow tank to fill up, to much pressure in your system from the coolant boiling would.
Have you checked your oil for coolant? does the car seem low on power? does the exhaust smell funny or have a certain hue?
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