just a theory on your 3rd gear pop out but, I took the cover off of my 4 speed to find that the trans 'holds' the gear by using a ball with a spring under it that fits into one of three notches on a slider rod. This creates the 'click' when you put it in gear. My particular trans had been modified, looking like someone had lost the ball/spring/both at some time. My trans no longer clicks for 3rd or 4th gear making me think that it would be likely to let go under load.
The other fellas that commented are true veterans so take my advice with a grain of salt
