Mitsubishi Repair: gears slipping in overdrive only, mitsubishi magna, transmission shops

Question
My 1996 Mitsubishi Magna (automatic) slips into neutral constantly as soon as it's warmed up, and will engage for a few secs after putting back into Neutral then Drive again, and I was told last week by a mechanic it needs a whole new gearbox, but after some net research I tried it out with overdrive off and it's completely fine (apart from using more fuel on the freeway of course). Do i need a new gearbox?

Answer
Tammy,
It's hard to say if you need a new gear box without a complete inspection of the transmission and also an inspection of the transmission computer.  It's possible the problem could electrical rather than mechanical.  I'd recommend you get a few estimates from other transmission shops before replacing the transmission.  
Good luck!