Putra Gearbox giving up ...

Lately I've found out that my putra's 5th gear is giving up... When I'm driving at constant speed at highway , sometimes the gearbox will kick the gear out from the 5th gear position into N automatically. Its kind of irritating, when the first time it happens, i thought my clutch has failed, because the RPM keep hiking all the way up but the speedo is going down. Checked with my Mechanic, he told me that the 5th gear need to be replace would cost around RM3xx and a whole day work. Guess i will fix it together when I replace my clutch pads. Kind of worry my other gears might be spoilt also, because the other day driving at 4th gear, heard some sound like gear grinding. I wonder how come problems happen on the 4th n 5th gears as I normally trash my 3rd and 2nd gear only, when I downshift .... LoLz... anyway the putra gears are kind of old, need some new replacement parts on it d i guess. Travel more than 265,000 Km already.