This happened to me on my 91 3.0. My mechanic changed out the thermostat and it worked! Turns out the temp switch mentioned above regulates when the transmission will go into overdrive. It won't do it until it has reached a certain operating temp. Hope it is something as simple as that. Good Luck!

1989 Mitsubishi Montero SWB V6. Torsion bar crank, Gen II springs, 31X10.5" BF Goodrich M/T's
2005 Mitsubishi Montero Limited 3.8 V6
2011 Subaru Impreza Outback Sport