Of course you are taking the readings right off the battery poles and not from the cable clamps, right? Bad connections are the first suspect. No chance of a slipping serpentine, right? That's the second culprit with an intermittent charge light. Assuming you want to do a parasitic check, I PDFed the pages from my GM service manuals. You can download them here:
http://208.255.159.239/drain.pdf but here's a snap of the critical calculation for current draw depending on your battery specs:
