Overheating

My SUV is overheating when I do not run the heater. I have flushed the system, replaced thermostat, replaced temperature sensor and gauge with aftermarket one, replaced radiator cap, and replaced water pump. Nothing has helped at all. It will run idol for about half an hour or if I drive it about a mile or so it runs normal and then it very rapidly goes to hot. I can turn the heater on and it very rapidly cools back down. I am at a complete loss so any help would be great. The truck runs great it does not smoke, tap, or knock. This was just all of the sudden and no warning. It does not leak any fluids at all and I am just not sure what to do next so like I said. Thanks in advance for any help you can throw my way.

Thanks. I will give that a shot and let you know.