I know you guys have a lot more experiance than me, however I do remember the same problem on my 53 Suburban. I was driving down 5 with no money in my pockets, I just got out of the service and had everything I owned in the back. The transmission kept on popping out of third gear. I tied it down with a rope and held on with all of my might for about 200 miles before I gave up. I replaced the transmission with snow on the ground and 22 degrees. It wasn't until I got home and took apart the transmission that I found the problem. The side plate cover was worn really bad. I replaced the cover and that fixed the problem. Another cause can be the pilot bearing. Just my two cents. ......g
