[ChevyGuy]

11 or 12 volts is no good, it should be 14.2 volts

It won't be that high after turning on the car but before starting the engine. I'd expect 12 something. 11 is too low. With engine running, you should see at least 13 something (but right after a start it will be closer to 14).

But yes, a bad battery can be flaky. Not all testers can spot a battery that's gone flaky.