1994 Mitsubishi Expo Transmission unexpectedly down shifts

First, I think that my vehicle is the 2 wheel drive model, but it might be all wheel drive.

My 94 Expo's transmission will decide about every four to six months to randomly down shift, sending my RPMs through the roof. When this occurs, it feels like my brakes are being applied, even though my foot is always on the gas when the down shift occurs, and I can hear a squeak as if I were trying to 'peel out' of a parking lot.
It occurs between the speeds of 25-35 mph. Last February, I lost 5th gear completely. I have taken it to a repair shop and they said that they could test the electronics, but that minimum cost to fix the thing was going to be $1,400.00. I parked the car to save up the cash to fix it, and when I got back into the thing to drive it to the shop, it was working perfectly.

So, do you have any ideas as to what could be causing the change in transmission performance?