Since I've owned the car, when it is relatively cold out (i.e. under 60F) and the car sits for an extended period of time, it starts up fine then after a second or two starts to idle too low and backfires, sputters, chugs and sometimes stalls. The idle will occasionally rev up to 2-3k then drop back down again and almost stall again. If I hold down the throttle, I can hold it at 2k to manually idle it, but even then it surges a bit even though I'm keeping the pedal perfectly still. I've already replaced the idle air control valve and tried running fuel injector cleaner. It's a 5.7 LS1, automatic, with about 127k on it. It's almost entirely original, but I did have the alternator rebuilt a few years ago and also replaced all of the pulleys/belts/tensioners along with the battery. The previous owner also replaced the EGR valve just before selling it. It does have quite a few CEL codes stored, but I haven't had a chance to get them read since I took it out of winter storage. It runs fine once the car warms up for about 30 seconds to a minute. I'm leaning towards some type of vacuum leak (maybe one of the manifolds?) coolant temperature sensor, or perhaps an o2 sensor is going haywire. I believe these also have some type of air pump to supplement air at cold startup to keep it running rich. I seem to remember a code for this as well when I did have them scanned a year or so ago. Any thoughts or tips would be helpful before I start dumping more money into replacing a bunch of things that may not really be necessary. I'm much more familiar with Hondas/Toyotas and I know on older Hondas for example it was usually either the Fast Idle Thermal Valve or the Idle Air Control Valve, but even then it usually wouldn't act like it would want to stall, the idle would just rev whenever it was in park. Thanks!