I agree that an additional radiator will probably only add a little time to how long it takes to reach the high temp you are reaching. Your going to be pretty limited as to how you can add another radiator and pass enough air through it to make much difference. If you could add another cooling feature, I'd see if you can plumb it as a reserve to tap into when you need it. Adding a couple of quarts of cool coolant to hot system will bring down the temps for a while, maybe long enough, if your running close to range.

How hot is it getting? 210-220 is not out of an allowable temp range under a 16 lbs pressurized system. Gene