Mainshaft/Countershaft bearing. Happens all the time. Try the fluid fix first, then open the tranny up and look at the two larger bearings. You will see it right off if the bearing has fallen apart or is real sloppy.

 

i had a bad intermediate shaft seal that leaked like crazy and when my tranny would get low 5th gear would slip out... never had a problem with any other gear than 5th though..

 

well i did the fluid and it is still doing it but not nearly as muc.....????

 

probably bad syncros in that gear... DOH!

 

 

does it grind bad or anything when going into that gear.
probably the fluid like every1 else says

 

Sounds like a bearing problem. Happened with mine too. Ordered all the bearings from honda. New tranny for 150 hell yeah.