Re: EGR valve
I don't think your problem is with the egr or o2 sensor. true a clogged egr affects idling, an 02 sensor adjusts richness/leaness and both work in somewhat a harmony, along with the rest of the fuel system. My 1992 uses same vortec engine and just runs like the devil with 185,000. ever had your injectors cleaned professionally ? A constant clogged egr probably means your engine is not burning off all its gas efficiently, meaning your problems may lye in sensors, rings, or valves.
Just my opinion, not a pro.