My truck has been running hot for a while now, I have replaced the thermostat and had the radiator flushed. It runs hot when on the open highway and does fine in town driving. If I cut the a/c off when it starts to run hot and slow my speed down a little, the temp will drop a little. I can get it to cool down before the gauge gets into the red. What can be causing this?
Providing no one touched the cooling system before the problem started, then we can rule out the system being air bound. Make sure the belts on the water pump are tight and are not slipping.
If the system hasn't been open before the problem happened then the first thing to check is to see if the gauge is working properly.
This can be done by having someone hook up a scanner and look at the engine temperature and compare it with the gauge reading. It's possible it is just a bad connection at the coolant sensor or a bad sensor itself.
There is still a possibility that the radiator has a blockage in it and it is stopping the cooling system from performing properly. You said that the radiator was flushed, was it done at a radiator shop? They will be able to test the radiator to see if there is a blockage.
When the truck runs, hot try turning on the heater. The air flowing through the heater core acts like a little radiator and will help cool down the system. If this works, then the radiator should be checked for a blockage.
I don't know if your truck was equipped with an air dam or plastic shield under the radiator which would have been mounted on the bottom of the radiator support. If this was broken or torn off then this could cause the temperature rise on the highway. This is just a guess but check to see if any thing like a shield or deflector was broken off the radiator support.
See if these suggestions help.
If not then supplying us with more specific information that would include the engine size, transmission and if it is a 4x4 will help us help you better.