If 2 radiators cracked in the same place I would look at the subframe flexing....the car will overheat and go into limp mode well before the radiator crackes for being overtemp. I think the subframe is flexing and exerting pressure on the radiator mounting points and causing it to crack. No coincidence that it idled fine in the driveway for 20 mins and then cracked in the same place 1/4 mile down the road. can you post pics?