1998 Ford Windstar Radiator fluid is a rusty color?

My van is a 3.8L and have had to have the motor replaced about three years ago. Ever since we have problems with the cooling system. Firsts it was the freeze plug paid to have it replaced. Then it was a pipe going to the manifold had pin size holes in it, payed about $400. The shop said they replaced it. Strangely both of these things have malfunctioned again and had to be repaired and replaced. Now I have a new problem, I don't know what to do. The fluid in the radiator is rusty and it bubbles into the coolant reservior. This is our only means of transpotaion and really cannot afford to have another big repair. Any information will be a big help.