[69 Roadrunner Nut]

Have a 69 roadrunner and the transmission has started slipping from second to drive (automatic) and then will stop and slip in all the gears when warmed up. The car will eventually stop pulling at all. If you turn it off let it set for about a minute start it back and put it in gear it will take off. Has 3600 stall converter and manual valve body. If anyone can tell me what the problem may be I would really appreciate it. Want to get her ready to drive to car shows. All help will be appreciated!!!!!