I agree to check your cap because that's the easiest route right now. Also, check to make sure your hoses are alright. Does your temps stay up while you're on the road or is it basically when you get off of the highway,etc and start to cruise (when the engine is all wound up).My 560 was doing the same thing when I'd shut the car off and would make the same sounds you could hear in the cabin.
I changed the coolant expansion tank (because the neck actually did have a slight weak spot and I'd see water bubbles seep from the old neck from the pressure). I also changed the thermostat, and finally I replaced the radiator which solved the high temps and she's not gone above 82 degrees since.
I thought it was the radiator but was reluctant to believe it because it was a "new" radiator that I'd installed not even a year ago.Check the little things first and then consider the radiator not letting your coolant circulate properly.Feel the radiator for cool spots after you take her off of the road. Also, make sure that the system doesn't have air in it . Hope its something minor but you never know.Good Luck!
High Engine Temp! Need Help Asap Guys!