well if it came out of a 98, then most likely its the head gaskets. Did you do the head gaskets your self? I had to a while back and subaru was giving out faulty head gaskets. If you can remember if they were a 3 ply head gasket, then they are defects, you need to get the newer 4 ply ones. But when the 98's have a blown head gasket, water doesn't go into the cylinder, the piston forces part of the exaust into the cooling system. Hope that helps.
(still a newbie for some d*** reason)