I had the same problem with trans shifting into o/d and after about 10 seconds the converter would come out of lockup and trans would downshift into drive. After two trans
rebuilds(complete),new pcm, valve body,converter,gearset,sensors,ect., The front pump which always tested ok, blew out. After replacing the pump, the trans has worked perfectly. The trans mechanic was amazed. I just wanted to give my situation and fix because I put up with this problem for 2 frustrating years. hopefully,this might help someone out.