Have one of the local implement or equipment dealers do a chemical analysis on your anti-freeze. I'm going to guess you have a bad head gasket or a cracked head or something allowing combustion gas into the anti-freeze and the anti-freeze pH is very low. I had the same thing happen to me on a 360 and it turned out to be a crack behind one exhaust seat that had been installed.

"We live in a time when intelligent people are being silenced so that stupid people won't be offended".