High idle, low idle, shuts off

This problem has been intractable. Or maybe it's more than one problem. Since purchasing the truck 10 years ago it has had a high idle. On starting the engine will pull 1,000 up to 1,500 rpm. Putting it into reverse to back out of the driveway drops the RPM down to 700. But it returns to high idle in drive. After driving for a while it will sometimes drop down to 600 RMP at idle, but start sounding a bit rough. The lower it goes the rougher it gets until it dies. So maybe once a week it will die at a stoplight. It requires full throttle for a restart and one foot on the throttle, one on the brake to hold the RPM up in order not to die again. Running, in park, it often searches going from 700 to 1000 RPM. Replaced PCU valve (GM), coolant temperature sensor feeding ECM, 02 sensor, idle air control (gm), throttle position sensor (gm) EGR valve (gm), EGR vacuum switch (gm), MAP censor, distributor pickup, and rebuilt throttle body including pressure regulator. Thoughts?