Did you buy the car used? Any chance it could have been a salvage title?
Could it be the computer?See if your dealership will replace the negative battery cable. While the symptoms you're reporting aren't listed in the bulletin they are sufficiently odd and random to warrant attempting a known simple solution to other strange electrical symptoms.
Even if it could be one of the processors on board, I think replacing the negative battery cable under the special coverage bulletin #14311 is the proper first step. Given that your battery tested fine, this should be the next step in troubleshooting your problem. The negative cable has a connector crimp that can go bad over time creating intermittent loss of grounding for many different systems. There have been so many bad cables that Chevy has issued the special coverage for them.Could it be the computer?