Coolant reservoir steaming/leaking from top?

I just recently bought this car; it's been sitting for the last 5 years approximately. When I bought it from the owner, he informed me it needed a thermostat, which I put it myself immediately after purchasing the car. After putting that in I bled the radiator system for air. I then noticed that when I drive the car, the coolant reservoir is steaming. I can also hear it bubbling in the reservoir and a little bit of coolant leaks through the top of reservoir. This only happens after the engine has heated up or the car has been driven for a few minutes. The engine does not overheat according to my gauges that function fine. I also have heat as well coming through the system. I also confirmed that no radiator hoses are loose or leaking. The guy I bought the car from said that it was something wrong with the radiator cap, so he bought one and replaced it before it was parked 5 years ago. But from what I've researched a lot of people are saying it's the radiator cap. Engine only has about 10,000 miles on it. The only other thing I've noticed wrong with the car is the idling. When the car is first started the engine will rev itself up automatically. It usually does this for about a few minutes and then idles normally. I cleaned the Idle control valve and that seemed to bring the revving down a lot. I don't know if that could be related but thought I'd add it. Any help is greatly appreciated.