While driving my 2004 Montero limited a few days ago at highway speeds it started to hesitate and sputter, like a engine miss or it being out of fuel (tank was full). I pulled over, let it sit for a few minutes, started it and drove it home with it running fine. The next day the vehicle ran fine for a couple of hours then started doing the same thing, it seems at 3000 rpm or higher is when the sputtering and hesitation occurs, at least when I start it and rev it in the driveway. I removed the radiator, accessory drive belt and associated pulls and covers to view the time belt and timing marks, they are dead on and the belt is in great shape. My thought was the timing belt skipped a tooth, well it did not. my next thought is a bent or chipped valve, but as I think more about it, why would it run great for a while them start sputtering, if it was a valve issue wouldn't it run bad all the time? My thoughts now may be a fowled plug, bad coil or something with the fuel pressure. Anyone have any ideas or faced the same issue with a resolution? I should add that I personally replaced the timing belt and head gaskets a few months ago and it has been running great since until the issue on the interstate a few days ago.

 

It is likely that this is related to a fault in ignition or fuel system. You can see if the PCM stored any Pending Codes. The codes can be stored even before the Check Engine Light goes ON. If the code points to a miss on a single cylinder, - look at ignition issue or a bad fuel injector. If it's a missfire is on multiple cylinders, I'd be leaning towards a fuel problem. This can be a clogged fuel filter or a fuel pump that is ready to fail. I'm more inclined to suspect the fuel system, since you said you observe this condition above 3000 RPM (higher load = higher fuel demand).