Do not rule out the radiator. It may look and seem ok, and you even talk yourself into saying but it's only XX years old as you did. But I am telling you a buddy had it happen and just this year with only 3 years on mine it happened to me. Radiator was going bad. Swap it out and all is good. It sounds crazy I know, but its the truth. Mine after driving awhile, especially highway speeds made the gauge climb --it would never over heat, but it got warm at 230-235 with the A/C on. It would stay 210-215ish with the a/c off.

In talking to some rad shops, they all said and agree don't expect much out of these newer "cheap parts store" radiators. I personally felt cheated only getting three years out of mine--of course that is all the warranty is good for on the one I bought.