you just removed the t-stat?

seems to be your problem right there. running without a t-stat will cause it to happen if you sit still or run your car for an extended period of time. the coolant just constantly flows and once it all starts to heat up the only way it will cool off is thru the fan and since its constantly flowing it won't cool like its suppose to.

put a t-stat back in so the car can regulate the hot/cool coolant.