Yesterday I drop up a local mountain to hang out with a few of my friends and I noticed on the way there, the car was running much hotter than usual. As soon as I shut the motor off, the coolant started boiling from both overflow and radiator cap. I thought that maybe the thermostat is not working so I tested it today, and it works fine. I changed the radiator cap, and that works great now as well, but the car still starts to overheat as soon as I start going though gears fast. The oil and coolant don't have any traces of mix. There is no water coming out of exhaust. What could it be?