GM transmissions of that era used to jump out of gear for two reasons in the transmissions. The shift forks wore out and the synchronizer assembly wore out. I grab every synchro assembly I find on eBay. 53 had 2 lugs and 54 up had 4 lugs. Did they replace these parts? A synchro assembly is about $125 and the forks about $25. A warped clutch housing or worn out pilot bearing can also cause them to jump out of gear. Of course the first thing to check is the simple things like linkage as mentioned earlier.