It does indeed sound like a blown head gasket. Check your radiator resevoir while the engine is cold. Make sure your radiator is fill as well. Note the water level in the resevoir. It should normally rise a couple inches or so from the cold level mark when the engine is hot. Check your oil as well. You are looking for two things: Abnnormally high indication (oil level should NEVER rise between oil changes unless you are adding oil) and signs of water moisture on the dipstick. Also, if you have water in the oil and the engine has been running, the oil may look like a milk-chocolate fluid...this is due to water being mixed into the oil. If you have any of these signs in your oil, this is a sure sign of a bolown head gasket and you should avoid running the engine until it's fixed, as this will cause severe damage to the engine. If you have no signe of water in your oil, start your engine and drive it until it warms up or starts to overheat. Look at the resevoir level and listen for sounds of water boiling into the resevoir. If the resevoir is significantly higher than normal or you hear boiling noises (popping sounds made by the water in the radiator/engine as it releaves into the resevoir), then you MAY have a blown gasket. Change the thermostat out first, though. This can cause these symptoms as well. And engine that is running too hot may not run well due to sensor input into the computer. A thermostat and gasket will run you about 10 bucks at the parts store. If the thermostat doesn't solve the problem, you may indeed have a blown heaad gasket. I had a car with a blow head gasket that would idle fine all day long. However, when I drove it, the internal engine pressures from the combustion processes were enough to displace the water out of the engine/radiator and into the resevoir, causing the coolant to overflow the resevoir onto the ground. This car did not have water mixing with the oil. Another car that I had with a blown head gasket caused water to mix with the oil, causing the oil to froth up through the throttle body. Good luck. Here's hoping it's just a bad therostat!