99 Ranger, 3.0 Flex Fuel, about 170,000 miles.

Started to overheat at highway speeds 65-70 mph. If I drove in town all day, stop n go traffic, took much longer to elevate the temp, would eventually raise temp, but no nearly as fast as on highway.

Changed the thermostat & did a coolant flush myself few weeks ago. Did not seem to help. Still had same situation.

Took to shop today and had them change water pump. Again, they brought it up to temp...no overheating. I drive home (after their closed of course) and it overheated quicker & hotter than before the water pump was changed. I checked around water pump gaskets for leaks and did not see any. Radiator cap was whistling & overflow tank boiling when got home.

What the hell is causing this? Is next step new radiator? Is it simple as just a new cap??

Any help is appreciated. This is driving me nuts.

Thanks,

Jeff