It is not the cap as it is not losing any water. A friend just had a very similar problem with a Jeep. The temp would continue to creep up at highway speed. The radiator shop indicated the flow through the radiator was 11 gpm versus spec of over 20 gpm. The shop rodded it out and now it does about 30 gpm.

I might bring the radiator to that shop for a flow test.

I was wondering if anyone had seen those exact symptoms caused by a radiator with low flow.

thanks
Joe