Your description sounds like my 90 Montero a few years back. As it turned out, the issue was with the ECU. I got it rebuilt and she is still running strong. If there is anybody close to you with a running 91 see if you can borrow the ECU to test your Monty. I believe the 89-90 are different than the 91. (is this correct?) So you need to make sure you get a 91 to test with.

This is where I got mine rebuilt
Av-pro

Hope this helps.


1990 Montero LS "Bottom Feeder" LWB, 33's, Cheapass Lift, 2" body lift, Dual BoUnCyS, Dual LSD's, KC's.
1995 Montero SR Complete LS interior swap and a BoUnCy from a 96 SR. General Grabber AT2's 33X12.5. 1.5" Body Lift