I have a problem with my truck consuming coolant. It takes about a gallon every two weeks. If I start the truck cold and run it for about 30 seconds, shut it off and pop the radiator cap, there's pressure that needs to be released. That shouldn't happen unless it's hot. It starts to overheat, but it cools very quickly once I start moving or I hold the engine at higher revs. No problems overheating while at constant speed. Bubbles come to the top of the radiator while running, but there's no horrid white smoke out the pipes and there's no water in the oil. What the hell is going on????