2006 Chevy Impala SS - Weird Problem...PLZ HELP!!!?

[Markus]

I'm having a recent issue with my 2006 Chevy Impala SS. About 50% of the time it will cut all power once i switch the engine off. The radio won't stay on..all power door locks disabled..etc. Sometimes it also locks the key in place until it randomly decides to power back on (takes anywhere from 2-10 min for this to occur). When the power does come back, everything must be reprogrammed. All the radio stuff, time, date, etc. As well as the compass must be re-calibrated..which is a pain to have to drive in circles sometimes twice a day. Is this common? Any idea what I could be dealing with here? A couple people suggested it could be a wiring issue and other suggested a battery issue. So I took it in to my local Advanced Auto Parts to have it tested out. The guy said the battery tested out good and that the alternator wasn't re-charging the battery properly...which seemed odd for car with less than 60K miles. He quoted me $300 for the alternator (5.3L V8) and I then left and bought one off eBay for $40. Good thing I did, cause I'd be pissed - THAT DIDN'T WORK!! Any ideas on what I could be dealing with here?? Any help would be GREATLY appreciated!!