it could b short, or a bad a b sylonoid which controles 3 4 shift, u need check ur line pressure to 3 4 drum sounds like losing pressure third and fourth are the same cluth, only fourth works with the 2 4 band also, hate to say it but prob need rebuild 3 4 drum again, ask ur builder if he put the overrune springs back in 3 4 drum, if not thats ur prob, its recomended to leave them out on the older 700 r4s but not on the 60s