It may not be a big problem at least I hope not...mecanic or anyone that has knowledge on cars:((?

I have a 2001 ford zx2 (2dr model of escort) it has 101,000 miles on it the oil change is up to date,I get plenty of heat and the ac works wonderful gets super cold,the antifreeze does boil a little in the bottle sometimes,it is brown but it does not go bone dry,the needle wonders over to the hot side some,but the thermometer never gets red like the engine is over heating,and also if i turn on my air full blast the needle goes back normal....what maybe wrong wit this car geesh cars i tell you!!!