Syncronizers can cause this issue. They are internal. However, they usually cause a grinding when shifting.
As far as it jumping out of gear, there are several things that can cause it to happen. Here is a list of things I can think of that you should discuss with the mechanics that did the work.
1) Worn Pilot bearing
2) Bent shifter fork
3) Worn gear teeth or face
4) Excessive gear train end play
5) Worn Synchronizers
6) Missing dent ball spring
7) Shift linkage worn or out of adjustment
8) Engine / trans bolts loose or out of adjustment
9) Transmission alignment
Of all the things I listed, two things stick out. First, the shifter linkage (since it was checked by more than one tech). Also, excessive geartrain end play since it only happens in 5th gear. Also, because of the high mileage, the gears themself could have excessive wear. One other thing that is easy to miss is the dent ball spring.
Other than the things I listed (as long as you hear no bearing noise), there isn't anything that should cause it.
Let me know if this helps you or if you have other questions. NOTE: Everything I mentioned requires that the trans be removed again other than the linkage adjustment.
Friday, February 11th, 2011 AT 9:37 PM