Scorpio MHawk-5th Gear Slips to Neutral

I used to boast,that it was been niggle free all this whilelol: It seems a major one is here to haunt:Frustrati It has done around 24.5K kms. Recently took her on a 600kms round trip and she completed that without any fuss,and i parked her once i was back. As i took her out in the evening and shifted to 5th,it slipped to neutral. I ignored it,thinking,that i might not have slotted it properly. Then again after a few kms,i shifted and it again slipped. Now that made me worried,and as i rolled in to a toll booth,i heard a faint hissing sound from below.I pulled over,and it went away after like 2 minutes. The gear slips if i upshift at 60-80km\hr,if i upshift at speeds in excess of 80,the gear doesnt slip,and to be on the safer side,i brought her back home in 4th gear. I did some research regarding this,and found that m_upreti had to replace the gearbox due to this. Guys any more info,if it can be due to any other reason?

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