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Pathfinder(1988) stalls after starting for 20secs

1988 Pathfinder (125,000 miles)- After starting, runs smooth at idle for say 20 secs or so. Then when applying throttle revs a little say 2000rpm then cuts out. Also, if I do not apply throttle, after 20-30 seconds it starts running rough stumbling and then stalls out. Plugs are very clean. Injectors puff fuel. Angle sensor (rotating by hand over range of distributor) pulses ~5volts and puffs injectors. The other lead varies voltage 1o2volts to ~4volts rapidly. I haven’t hooked up a oscilloscope yet. The AIV exhault recir has gone out. I’ve connected the exhaust hoses and plugged the vacuum line (exhaust goes into throttle body). I’ve plugged the exhaust recir also (no exhaust goes into throttle body). Symptoms are the same. As the throttle position sensor is at back, I haven’t checked. Note: The ECU doesn’t have a rotation mode selector and I haven’t found the way to select the mode and so do not have that the trouble code.

What is the likely cause that makes this otherwise good truck undriveable. ie. The most common causes and weaknesses in the Nissan system and suggestions for trouble shooting sequence.