On occasion my stiebel box will pop out of drive while driving at highway speeds. It never does it while accelerating ar city driving.  And sometimes it'll go a month without doing it then jump out 3 times in one trip.  The ball and spring on the slider are fine and it has a new shift fork. has anyone else had a similar problem ? Does the air cylinder actually hold it in gear or just slide the fork ? I'm thinking I may need to replace the slide gear and drive gear in the box. 

You can check The Sliding clutch( gear) Did you check shifting cylinder and piston ?

There is nothing obvious wrong with the shift cylinder and it seems to move the slider from pump to drive without any problems. Does the cylinder actually hold it in drive with pressure or is it just there to move the slider and the ball and spring keep it in drive ??

The shift cylinder is held in either position within pressure, doesn't sound like you have a problem with your air system, have you replaced the cylinder seals?

if The seals is damaged you will find an air Leakage from The Solenoid 

I would be looking for some sort of vibration in the drive line, or rear ends.

Concrete7111,

Heres is a idea and this is what happened to me, but it was several years ago. (10 or so)

1989 36 meter 14 cell putz - it would jump out of gear while I was driving. Just wierd I thought. Went through it

checked for air link, found none. Replaced shift fork, because it looked somewhat wore out. Ran the pump and it did

the samething again with the new shift fork.

So, we tore into the the stiebel box found nothing odd, replaced the shift fork and put it back together. Samething

ran it and it would jump out of gear.

My mobile mechanic at that point said I can't help you anymore I am too busy, BUT my brother will come and help

you. His brother and I tore down the stiebel box again. BUT we looked very closely at all the bearings. Those

bearings felt good to me, BUT the mechanic helping me, said NO. We need to replace the bearings in the Stiebel

box AND the pump was fixed....

Long story short - My stiebel box needed new bearings to fix my problem with the pump jumping out of gear.

It was a lot like you. Once a month the pump would jump out of gear. Then once every two weeks. Then more often

and often.. Good luck - Sam Woodbridge