Hope this is the right place for this post. My ''75 F250 with 360 and C6 slowly took longer to get moving once started. It finally refused to move at all in any gear. Trans shop told me it needed the C6 replaced with a rebuilt unit. Instead a buddy and I tore it down and rebuilt it. Once we got into the trans it looked pretty good, seals were not hard, etc. Installed a rebuild kit and new torque converted anyway and put it back together. Test drive went well the night we got it running. Next day we took it for a longer (5 mile) drive to a gas station and it worked great. When we went to leave it would not move. Pushed it out of the way and puzzled over what was wrong. After 15 or 20 minutes tried it again and it worked, drove home without trouble. Pulled in to the shop, shut it off and started it again. Would not move. Next day tried again, after it ran a couple of minutes it would move. Shut if down and let it sit a few minutes and checked the fluid level, it was up a couple of inches on the dipstick. Got it running and after several minutes checked the level again and it was where it should be. We think the torque converter is draining down.Any thoughts or suggestions?

As a note, prior to my owning it someone had put a shift kit in it as they used it to pull a large trailer. Shifts would snap your neck, and we think the trans was rebuilt at that time given the condition of the seals. Had ordered a new shift kit to try to get back to something a bit milder, however it had not arrived in time so is a project for another day.