This may be way off base for a Mitsu (and I am a noob to the montero) BUT, my brother has a 1999 F-150 4x4 and he started having some bizarre auto trans shifting problems last year. Trans shops would put it on the lift and check out things, ohm out the solenoids, etc. All checked good. It only affected 4th and lockup, if he drove in 3rd, no problems so he just figured it was getting close to rebuild time. Skip forward a few months and it was time for new spark plugs and he noticed that one of the plug wires didn't look so good and it ran right past (and along with) a wire harness loom which happened to be the feed down to the transmission, so he bought new, quality plug wires and made sure to run them all nice and neat and kept that wire from rubbing the wire loom. Since then he hasn't had ANY transmission problems in 4th/lockup and the only thing we can figure is there must have been some secondary ignition voltage bleed into that harness causing the solenoids to go a little wacky. He told the trans shop about it and they have called him back to tell him they've fixed two other trucks with similar symptoms by putting new plug wires on. So it may not hurt to check that any wire looms are a good distance from any spark/coil wires.