My 2003 GMC (owned since new) with a 4.8L V8 and automatic trans has started stalling sometimes when the engine hasn't completely warmed up. The idle speed gets too slow, causing the stall. It starts up with no problem, idles fast enough for a few minutes, then gets too slow until completely warmed up and then the idle speed is fine again. No problems going down the road, just at idle. It has about 120K miles on it. No check engine light. I'm an old shadetree mechanic and I've adjusted idle speed on carbs and throttle bodies but I don't think this f.i. has any adjustment. Any quick fix or do I need to take it somewhere for diagnosis?