i have a b16 cable s1 tranny that pops out of 5th gear all the time.. low rpm, high rpm. it doesnt pop out of any other gear.. just 5th. It didnt do it when i first got it, but now it does it constantly.. i cant even drive on the freeway properly. it only does it when im on the gas. It stays in gear with no throttle, but when i give it gas it goes for a little and then pops out.. if anyone has any ideas please post. thanks.

 

My stock Si tranny did something similar when the input bearing went bad. It would pop out of 5th and 3rd gear, and sometimes it was hard to take out of or put into gear.
Have it checked out somewhere. Doesn't sound too serious, but it could turn into something worse if you let it go too long.