'91 S-10 blazer idle problem

[nightmare]

Hey everyone, I'm normally on the 67-72 side, but my dad has a problem with his s-10. When he does a cold start, it will idle to fast idle for a few seconds, then go to low idle like it should. But when it gets to low idle, it will idle down further like it wants to die, then goes back to low idle. It cycles like this until it is warm, then it idles like it should. We’ve tried replacing the MAP sensor, the O2 sensor, all plugs, wires, dist cap and rotor, all with no resolution. The next thing that he wants to try is to pull the gas tank, and replace the fuel pump, but it is getting fuel. Would a bad EGR valve do this? What about a bad catalytic converter? The truck has 200k miles, but runs like a champ when it does run. It’s the 4.3 TBI. Any ideas?