93 Suburban running rich, stumbles when hot

My 93 Suburban runs rich (black smoke from exhaust ) when cold, but just for a minute or so. Smoke goes away but it seems just a little sluggish all through the temperature ranges.
Hesitates badly under light throttle when hot but if you hit the trottle hard it lifts off pretty good. It did'nt pass the Ontario Drive Clean test because NO and (I think) CO levels were way to high.
Any ideas??? 5.7 with air. Thanks

My 93 stumbles at low throttle (1500-1800 rpm). I've read alot here, good info. Checked my distributor for shaft movement,it's good. Pulled the coolant temp sensor for the computer, and it ran better, but hard to start. Thought that was it, replaced it, did it again. Checked timing, it was a little off, set, runs better, but still does it. Today I was checking the EGR valve, can see it move under low throttle, blip the throttle and it doesn't move. Disconnected vacuum hose to EGR and it stopped. Plugged vacuum hose and EGR and test drove, it does not do it and runs great. I'm almost certain it is the EGR valve, but this critter has fooled me, so I think I'll pull it and try to clean it and try it again. Sure nice not to hear and feel the engine surge when you let off the gas. BTW I have a K1500 with the 5.7 liter, 350 c.i.