My 1986 4 Runner starts fine and runs great for about 10 miles. Then I get a loss in power. I can downshift and keep going. Then it will happen again in a short distance. Same thing. Also, when this started I noticed some exhaust smoke coming out the rear. This is a new motor, and even before that I never had an exhaust problem at all. The last time I drove it, the smoke came up inside the 4 Runner beside the drivers and passenger seats when I was losing power. This scared me so I just had it towed home. It had acted like it was running out of gas, too, but I had just put gas in it and replaced the whole fuel system not long ago. I was told it was probably the catalytic converter. Is this a probability?
If the cat is plugged the truck should run good or better cold then get worse as it warms up and the cat may get glowing red hot. So if you would go ahead and start it and then check the exhaust flow from the tail pipe then let it run 10 or 15 min and check to see if the exhaust flow get less and look at the converter and see if it is looking red hot. Let me know what you find.