S.O.S. 833 pops out of 3rd without load.

[FED 387]

Ditto on the stops/alignment of the shifter---also check the bushings on the trans shifter arms might be a cracked one----I think it also could be a shifter fork or a slider might be worn--- is it only third???? Do you have to "hold" it in gear to keep it from popping outa gear???