Transmission Problems....Need some help!!

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!!!!!

Check the fluid level. Also how long has it been since you changed the filter in it. I had one once that would not shift out of first and then would not engage any gears. Turned out the filter was so hard, no fluid made it through.