Is it my radiator cap?

SO...I drive a 1997 Plymouth Neon 4 door...I have been having problems with my car over heating recently. At first I thought it was the cool/heat gauge itself because my speed gauge goes out sometimes. So it was showing that my engine was running hot. Then I noticed that I had a trail of liquid coming from under my car to the back and around the corner of where I park. So I looked under my car and under my hood, and I saw that something was leaking. I thought it might be water from the AC drip line because I live in Arizona, and have been running it a lot since it is 115F here regularly. But it was a huge puddle. So I stuck my thumb in it and tasted it, and sure enough it was super sweet, so obviously it was engine coolant. I spoke with a friend who is a mechanic and he says it might be the radiator cap, because that is what allows the flow of engine coolant from the reservoir into the engine and back. The weird thing is, is that if I just run to the store that's a mile away, it doesn't over heat, and the reservoir has just a little in it. But if I drive say 5 plus miles, the car starts to over heat, and when I stop to check the engine, the reservoir is full of engine coolant and is boiling. So I think that what is happening is that when I drive a short distance the engine coolant is staying in the engine, but when I drive far, its being let back into the reservoir and not back into the engine. Which would in turn prove why it was leaking, or more so a over flow from the reservoir when the engine coolant was let back in. The reservoir itself isn't leaking, I have already checked. So it must just be an over flow. I also don't think that its the thermostat because from my prior knowledge, if that had malfunctioned, wouldn't it just turn all the way open so that the coolant can still run throughout the engine? I'm not a mr.fix-it on cars, but I would like a few other opinions please.