1996 cadillac sedan deville overheats on highway drives fine in town car runs good no missing put a new radiator and new thermostat but still overheats on highway gets hot and comes across dash engine overheating idle engine
I had the same problem with an Oldsmobile long ago. It would run all day at 55 mph , but would overheat quickly at 60+. It was a bad seal on the water pump . At high speed it leaked behind the fan and atomized the water so there was no sign of a leak except the radiator was low. I replaced the pump and it ran fine for years.
This won't fit in the comments, so:
A cracked radiator absolutely needs to be replaced, but it won't cause overheating until you're very low on coolant. BUT, that - losing a lot of coolant - can happen very quickly. I once trashed an engine due to a slightly-oversized hose clamp. It was probably leaking less than a cracked radiator would.
Now, on to cooling:
When you are at idle - say, sitting at a traffic light - the fans blow air through the radiator. Through the magic of heat exchange, the air flowing through the radiator absorbs heat from the engine coolant in the radiator, and your engine stays at a sane temperature.
At speed, the fan is ineffective for many reasons. The front end (and bottom of the front of the car) typically have big plastic pieces that act like a sort of funnel, sending air through the radiator. Since the fans aren't effective, you need this funnel / deflector / shroud system to keep things from overheating. Depending on the car, this may or may not involve shrouding and baffles that send air in the correct direction.
At higher RPMs, for an extended period of time, the engine is (duh) working harder and small internal leaks that might not otherwise present symptoms can become an issue.
Look under the front end of the car and see if anything is missing. Google for images of the front and bottom of your car for comparison.
Drain the oil, and look for coolant. If the oil isn't amber / honey-colored, brown or black, you have an internal leak that MUST be fixed IMMEDIATELY, or you're going to buy a new engine or a new car. If you see anything that looks greenish, you're looking at a bad head gasket or a cracked head or block.
Make sure that you have the AC off when doing a test drive. A problematic AC system can put a severe load on the engine, and lead to other issues - especially on a 23 year old car.
I might also do a coolant system pressure test and check for leaks, and a compression test.