94 Astro hard to start and will not idle

Astro Van 4.3L Z engine 220,000 miles

Drive the van everyday back and forth to work. Yesterday drove it home and ran fine. This morning it started poorly and when it did crank is stumbles badly and smothers out when I take my foot of the gas pedal. It smooths out pretty good at higher RPM (while sitting in the driveway. Have not taken it on the road). Will stay cranked when it gets up to temperature but still stumbles along like it is hitting on 3 cylinders. The check engine light came on once. I read the code and it was 45 (rich exhaust detected). Cleared the code and has not happened again. When I crank and run again for 5 to 10 minutes (at higher rpm to make it stay running) still no check engine light and no codes, just the normal 12. It acts like a tune up problem but why did it happen so fast (overnight sitting in the driveway). Went from running good last night to stumbling along this morning. Could it be the oxygen sensor? Where should I start? I have not removed plugs but I did check the distributor cap and rotor and while they are not pristine they dont look to bad. What should my plan of attack be? Thanks in advance for any help.

Update on the van problem. I cleared the codes and then cranked the car. Gave it enough gas to keep it running until it warmed up and it finally posted a code - 32. EGR valve. Took valve off, cleaned it well, piston was sticking, replaced gasket with Klean Screen gasket and fired it up. Runs like a top. This has happened before but it has always set a code. What had me stumped was the fact that it did not post a code so I thought that it must be something else. Then something in the back of my mind remembered that the engine and sensors have to warm up before codes are set.