Fifth Gear Pop out

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I noticed sometimes my car pop out of 5th gear. It never happened at high speeds and it has happened around the same spot twice. If I try to put it back in 5th it will pop back out. If I keep driving in 4th for a while then I can shift back to 5th. The first time after it happened I took the car on I-95 and drove in 5th gear for most of the trip (2 hour) and nothing happened. The clutch is less than a year old.

Any ideas or suggestion? I am hoping that is not a time for a new transmission. The car only has 145K