The Onstar module has a 15.5V lithium ion backup battery.
That could be causing problems.
Lithium Ion car batteries are better than AGM, but because of how the charging system
works in the Camaro, it won't properly maintain it.
However, it if were the battery winking out, your car engine would "miss", because the
coils and fuel injectors would wink-out as well. If the engine remains smooth running
when the wink-outs occur, it is NOT the primary battery,