It has been a while since I have had time to do anything on the forum. Way to much time at work. Seems like it I dont have a choice now. My 89 montero 2.6 with a rebuilt engine (about a year old) has been cutting out on me after I drive it for about 20 miles and is under a load. I changed sparkplugs to NGK (the ones I pulled out looked great though, changed them bcs they were autolites). Changed the Dist cap and rotor button. It is still cutting out though. I have used the stuff to get rid of water in the fuel lines also. I believe the thermostat is stuck open, I am changing that today and the fuel filter. I also changed the air filter. My only other thoughts would be the fuel pump or the coil. I also installed new plug wires when I installed the rebuilt engine. I did some research on this subject and found a couple of posts where some people had the same problem but there was no clear solution.

And there almost never is. There are frequently many parts involved in any given cycle of operation. Sounds like fuel pump or coil would be good to investigate. Also, ECU, ignition wiring, fuel return to tank (in case vacuum is being created).

Does it always only cut out after you've been driving for a while, or does it ever cut out when you've only driven it a short distance (a few blocks) from a cold start? If it's always only after you've been driving it for a while, I would tend to think that it's more fuel-system-related (i.e., fuel pump or fuel return). Although, I suppose some electrical components can act up also once things are heated up. Once your engine is fully warmed up, do you know for sure how hot it is running? If it's running really hot, maybe your fuel pump is vapor-locking.

Just some ideas. Hope this helps.

It has only been acting up after I drive it for about 20 miles or so. After it sits for a while it will do fine again for another 20 miles or so. I did not install a new fuel pump after the rebuild (I know - stupid ) I was planning on changing over to an electric pump. I also have not replaced the coil. I would imagine that it is the original. I really can't say how hot it is getting bcs of the thermostat is stuck open. So it hasn't been running hot at all. I am not losing any antifreeze either. It is running cool enough that I can open the radiator after a good drive and only loose about a cup of fluid and hardly no steam. I run a 180 thermostat, and that is what I will install this afternoon.

Next time it cuts out, remove your gas cap and see if there is a lot of suction behind the cap. And after removing the cap, see if it will fire back up right away or if it still has to sit for a while before it'll fire back up.

Once you check the big three...Fuel, Air and Spark start looking at the Vac lines and the thermo-switch. If you have a cracked or leaking vac line on one side of the thermo-switch after if warms up you could be getting a vacuum leak. Lack of vacuum causes everything from cutting out to lack of power. The vac lines like to crack on the underside where they attach to the nipple. The thermo-switch is the two port one, the thermo-valve is the five port one.

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I had an old carb'd Mazda engine do that once. it turned out to be a secondary ignition failure. It would idle smooth as silk, but try and go up a hill, or some other load, and it would stutter like Jerry Reed on crack.
Turned out I had replaced the plug wires with new ones, and one of the new ones was bad.
Check your cap, rotor, and wires.

Mine did that before I moved back to Virginia. Ended up being the coil it has ate up 2 in the 5 years I have owned it.

I took a long drive on lunch and it started doing it again and I did as one person requested. I took off the gas cap and there was no pressure or suction when I removed it. I think I will go ahead and get a coil this afternoon. I priced them at $20 at advanced auto. Probably needs a new one either way it goes.

Since I have never seen closure to this issue I will let everybody know that My monty is running fine now. I changed out the fuel filter and thermostat. Cleaned some gunk off of one of the temp sensors. I also unpluggeg a sensor connection to the environmental crap at the carb. I broke some of the tips off of the thermosensors when I was rebuilding the engine and the parts place wanted almost $100 for another one. So I just plugged up the vacuum lines and disconnected all the Environmental crap. Luckily in AL we dont have to pass inspection. So at least for now all is well. Gotta return the coil and fuel pump though. Want be needing it now. May keep it just in case though.
Thanks for all the help.