is yours a sentra or a 200? I know from the years 91-93 they suffer from a problem called fifth gear popout, but it does not sound like that, it also sounds to me like a busted syncro, maybe stripped, and at higher rpms just not catching like they should. if it is an older model, I would seriously consider buying a transmission from someone or somewhere, I have seen them go as low as 300-500 bux, almost double the price as doing it yourself, but you will not have to mess with it at all, and not have to pay someone some major bux to fix it either. sort of 'plug and play' instead of "crap, where did I put that retainer" or "well, I have six bolts and a few clips left over?"