A new puzzle on my 4.3 V6

Hey... thanks all you guys who gave me feedback on the issue on the 99066 recall. The consensus is that replacing the SCFI with MFI (per the policy) will be on my tab.
BUT... let me give you the latest. I took it in to a dealer a few days ago to have them check it out. They ended up putting it on some kind of analyzer and found that the timing was off. The mechanic said it read 30 instead of 4. So he adjusted it. Apparently, there is also a diagnostic in some GM system they use that told him to change settings in the ECU, which he also followed. When I drove it home, it ran like a champ. I tried to ask the guy what the readings meant and what was reset in the ECU (change in duty cycle?), but he couldn't tell me. I was cautiously pleased. Not for long - next day, I had misfires again, sometines worse than before.
The following day, something interesting happened. For starters, it took 6 or 7 seconds of cranking before the engine fired up. It ran fine on idle. Then I left to go to the post office (10 minutes, stop and go), coughing and sputtering all the way. I went inside for 5 minutes to mail some stuff, came out, fired up the truck... and it just purred. Not one hiccup all the way home. I've encountered something like this before: that is, it runs rough (misfires) until the engine is warm AND I turn it off for a short period (not enough for it to cool). Could it be that a sensor is malfunctioning when cold, the ECU takes a reading, thinks it's running rich and leans it out, causing misfires. Then, when the hot engine is restarted - and now the sensor is working properly - the ECU sees that all's well and keeps fuel at normal levels? Could it be a stuck EGR valve? I've seen other posts about stuck EGR valves on this engine, but not the same symptoms as I've been having. I did once get a code for the EGR pintle valve (not sure if "valve" is the right word). Not recently, though. Maybe a bad coolant sensor?
I plan to run a test each day where I start the engine and run it for 15 minutes, sitting in the driveway. Before I leave, I'll turn it off for a few minutes, to see if it then runs smoothly as I leave for work.
Thanks for any additional thoughts or suggestions.
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My Pontiac 6000 did the same thing, I changed the ECM and it took care of the problem

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