i was driving across a bridge going to the beach. the car pulled up the hill of the bridge just fine, but going on the downside maybe 50 yards over the peak it pops out of fifth and redlines and i havent been able to get it into 5th since. doesnt grind, doesnt grab, doesnt do anything. putting the shifter into 5th is just like pulling it into neutral. can cruise down the road off the clutch and in fifth. it doesnt have a pop to it like the other 4 gears. every other gear works fine including reverse. i did have 3rd pop out on me twice but not sure if it was because i was pulling on the lever to hard or the way i had my wrist or actually the trans popping out. needless to say i had a 2.5 hour return trip in 4th gear. fun 
any insight on this would be greatly appreciated.
also the bottom end of the block has just been built, but the motor held up fine. not sure if that would have anything to do with it but figured id throw it out there just incase.
add more oil to your transmission…works for me when i was slippin out of 5th gear…now it grips fine…dosen’t hurt to try.
i did drain and refill it today to make sure there wasnt any metal in it. came out fine no metal so im sure its not the gearing itself. what weight do you use in yours?
use honda mtf… keep it simple.
and yea you are low on tranny fluid.