What year/model is your Montero again? If it's a DOHC 3.5, then also inspect your throttle body butterfly valves. They like to come loose eventually (they use phenolic spacers which wear out and when that happens, they start to vibrate which loosens the screws holding the butterfly valves to the point that the valves come loose and are ingested into the engine which is a really bad thing.

I'd consider the possibility of replacing the intake after 150-200k miles. Unfortunately they don't sell the bushings and i'm not sure that any aftermarket options are available. I even have a spare used intake that I was going to explore the possiblity of making replacement spacers using a better material.
Unfortunately there is no hard and fast mileage when this happens at so that mileage number is a wide ranging guestimate based on when others have nearly or actually failed.
HTH.