Galant 95 Transmission problems

Hello,
I have a Mitsubishi Galant 95, Its had 2 transmissions replaced (rebuilt ones), Two years ago, the car started to skip on the first to second gears. I was afraid it was the transmission again. I took it to some mechanic and he replaced the "Overdrive assembly kit" and the car was working fine until now. It is skipping again into neutral when in low gear, I've taken it to several shops and they say that I need to put in another transmission. A used Transmission seems to be my cheapest option ($800 w/labor, done in 1 day. Does that sound right?). I'm not sure if I indeed need a new transmission and if I decide to go for the used transmission will it be wised? I appreciate any advice you may have.

Thank you.