There is the occasional shift linkage adjustment problem or a bad bearing, but in most cases transmissions jumping out of gear can be traced back to misalignment.
I would also like to know where the misconception came from that syncros have anything to do with a transmission jumping out of gear. If you were to say the teeth in the syncro drum were warn then that would make sense and could cause a transmission to jump out of gear. The only job of the syncros is to make the meshing gears come to the same speed for engagement.
The Chevrolet shop manual talks about the need to align the transmission to prevent a transmission from jumping out of high gear. If the front and rear shafts are misaligned it will cause the shift fork and teeth to wear. A transmission may last a long time misaligned but it will cause damage and eventually fail.
I have made up my own tool to check alignment and have found that most transmissions need shimming to correct the alignment.
I can help any one interested with the alignment process.
