2005 Mustang V6 fan cooling problem while idling

My mustang seems to not be cooled properly while i am idling. If i sit in my car and let it idle for about ten minutes the antifreeze starts to boil and come up through the cap. At first i thoungt it was just a bad cap so i replaced it. The problem has continued so i started looking around and now i am thinking that i need to replace the temperature sensor that turns my fan on and off. I was also wondering if anyone knows if the fan on my model has a hi and low speed? When i am idling with my a/c on i can hear the fan kick on about every thirty seconds and then it kicks off and i was wondering if it should be going to a higher speed if it has one? Also something else the might help is that when i idle the a/c will cycle from cold to somewhat warm and then work its way back to getting colder. I just thought that my fre-on was low so i hooked up a tester to my a/c line and when the fan is off the a/c blows cold air but the tester reads that i am low on fluid. When the fan kicks on the tester reads the i have the proper amount of fluid in my a/c but then it blows out warm air inside. I hope that i just need to replace the cooling fan temperature sensor to solve both problems but i have no idea if that is even the problem. I would really appreciate suggestions that anyone might have to solve these problems I'm having. Thanks