It could be the water pump, one way to check is to get the car up to temp, before it starts to puke out the over flow, feel the upper and lower rad hoses. If one is hotter than the other then there is some sort of circulation problem. I had the same thing happen on a 77 Olds Cutlass, it was acting the same as your car and it did turn out to be the pump. I put in a remanufactured pump and just my luck it was bad also, not even 50miles on it and the impeller quit spinning on the shaft.
If it is not the water pump then there is some other type of blockage.
Good luck I hope you track down the problem.
Oh yea, welcome to the forum.