First and foremost, check the condition of your motor mounts.
If the engine is not sitting correctly, it can put a strain on the shifting components, causing the car to pop out of gear...
I had a similiar problem popping out of 3rd gear, I knew it was because my engine mount (timing belt location) was bad... I replaced it with a new one, and my problems were gone...

My guess is it has to do with the linkage.
Check it out & let us know.