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The symptoms you describe are consistent with a vacuum leak. On the 4.3 vortec engine that you have in your truck, the usual source of the problem is an internal vacuum leak caused by the EGR valve not closing completely. If you have a scan tool available, take a look at the EGR position reading; it should be at zero when the truck is idling.
If you do not have access to a scan tool, take the EGR valve off of the engine and look at the bottom: you will probably find a small chunk of carbon holding the pintle partially open.
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