radiator cap keeps leaking?

hey my 99 gt's radiator cap was leaking out coolant, slowly spitting it out first, then after awhile a bunch flowed out, so i though bad cap, got a new one, still does it, what gives? the coolant light comes on because of the leaking, but it doesnt over heat, the worst it gets is the needle pointing straight up. im clueless as to what would cause this
 
Havent used water wetter but I have problend coolant in my system right now. about a 20/80mix of coolant/water...ive noticed it stays cleaner so its all good.

Weird about it spitting out coolant and not overheating. Did you put an overdrive pulley on your waterpump or something? :shrug: