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

[FED 387]

Well try the fork and side cover see if that works if not look at the 3rd gear synchro next--about the cover look at the hole the shifter fork goes thru see if it is wallowed out from use and check the fork to for excessive wear---are both forks the same if so reverse their positions swap one for the other see if that cures it--I'm still saying synchro is messed up some way