Lost 3rd and fourth gears they return after about 12 miles

Have the codes read.could be engine or transmission problems. It is in limp mode now.

replace the injector tips as they are squirting and not atomising. They will also be dribbling and that is allowing fuel in before the injection time

There is a leaking oil seal in the injection pump thereby contaminating the fuel.Usually the front oil seal.

The Transmission computer is the Achillies heel of many Mitsubishi vehicles , it sounds like your Pajero is in " limp home " mode which gives you third gear only.
See if you can borrow a replacement computer to try.
Mitsubishi scrimped on the quality of the capacitors in their computers and their failure rate is very high.
Happy hunting Marshall

i guess this is auto, if so its got dirty fluid in it, this means it torn a clutch, the box needs replacing.

check the throttle bearing, the teeth on the gears, or if not the through out bearing

Fuel pump: The vehicle is sensitive to fuel starvation caused by a clogged in-tank filter. (1991-96)

• try disconnecting the cruise control module as a test...
  
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