try this.

take off the bottom radiator hose, draining all the water.
stick the hose into the top of the radiator and turn it on
if the water does not flow into the radiator quickly i.e. if it overflows at all then maybe the core is blocked

a radiator should flow easily and quickly.... try it with the hose on one side then the other to see if this makes a difference

It's a simple test that only costs you a little time and water