If the trans is producing too much line pressure, the converter will push the crank forward, wearing the thrust bearing. If you have a large camshaft that lowers the manifold vacuum, it will cause hight torque convertor oil pressures. This will force the convertor forward. Also, check for a restriciton in the oil cooler. A restriction in the line or cooler itself can cause high pressures in the tranny, again causing the convertor to force the crank forward.