You may have TCU issues like Thom stated, but you could also be in need of checking to make sure the shifter is actually engaging all gears in the shifter range.
Start there, and the work your way forward. You could have valve body issues or internal parts issues. Valve body could have a problem in and of itself but if the filter is plugged, shifting becomes irratic, and why would the filter be plugged?
This is where in theory the internal issues could rear their head - ie debris from worn parts is causing fluid problems. If not these issues, then it could be an electrical issue with the wiring/solenoids in the trans.
IF you can get an online version of the factory auto user manual, or a paper version, then you can start trouble shooting.
There are many manuals located here:The Collection of gosh knows how many standard and auto box manuals, plus wiring diagrams, and complete car manuals for nearly everything Mitsubishi from that era.Engines and transmissions located in this folder
Take your pic.
I used these manuals years ago when I fully understood the auto trans with no sign of forgetfulness, and it got me to the point of condemning parts related to the torque converter lockup or possible valve body issues in the 3 speed auto with my old Colt. Fluid change would half sort it, then it would return within a few thousand KM's, according to the next owner. The issue was divulged at the point of sale.