2002 montero sport trans downshifts on highway

SOURCE: Warning light for 4-wheel drive

I had this problem on my 1995 Montero. The 4-wheel-drive indicator lights wouldn't stop blinking when I switched from 4-wheel drive to 2-wheel drive. It turns out that I had a slow leak in one of the front tires. After I fixed the leak and made sure the pressure was the same in both front tires, the problem went away. 
This made sense to me because if one tire is much lower in pressure, it's closer to the ground, i.e. has a smaller radius, and therefore rotates more quickly. The manual says to drive in a straight line when disengaging 4-wheel drive but with the lower tire pressure on one side it was the equivalent of driving in a circle. 
You could try checking your tire pressures and see if that makes a difference. Hope this helps. 

SOURCE: 2002 Mitsubishi Montero Sport Turn Signal Switch

i have the same problem in my 1998 Montero and was surprised to see your posting. I just went to an auto electric expert and diagnosed it as a broken turn signal switch that needs to be replaced.

SOURCE: Mitsubishi Montero Sports will not start...

I HAD THE SAME PROBLEM. I TOOK THE AIR FILTER HOSE OFF THE CARBARATOR AND ADDED CARBARATOR CLEANER. PUT IT BACK ON, AND WORKED. TRY THAT.

SOURCE: Transmission does not shift on "D" - '00 Montero Sport

i would check out the timing. but i little trick the i have learned was to put break fluid in your trany. it cleans out all junk inside

SOURCE: Gear Neutral light blinking while the gear is on

I believe it means that your transmission fluid is either getting too hot, or there's not enough fluid