Posted by gbh on June 16, 2005 at 08:57:56:
this is unusual in that most write about second gear jumping out. The responses to those questions typically address the shifter poppet balls and springs and also focus on the syncro rings. Maybe that somehow applies here too but I'd think the syncro rings would not affect first gear. You can find some of those post below within the past several months. I think I'd first examine the shifter if you did not rework it too. Sounds like it is not fully engaging and is being squeezed out under high load.
Good luck! Let us know what you figure out.