Ok. I recently acquired a 2000 k3500, 454 4l80e. 77k miles. I was having similar issues. Mine would idle erratic and occasionally stall, but only when warm. Cold it ran great. No codes, no ses light. Went through all of the fuel system (injectors, pres regulator, filter), did a full tune up, plenum gaskets. Monitor with my nemisys scanner. Could see nothing that would indicate the problem. Cam retard set at 0°. Ended up being that the torque converter was partially locking up at idle. Through some research I found it was a somewhat common issue on the 4l80s. Sonnax sells a valve that fixed it right up. And since I planned to drop the pan anyways for a trans service, it worked out well. Valve is easily accessible once pan is off, changed out in 5 minutes. Night and day difference. Part number is 34200-14k, lube regulated pressure regulator valve (aka line to lube valve). This may not be the same issue your having but man, it sounds very familiar.... Hope this was of help!