Hello, I bought our Lacetti on Wednesday this week, and we too have experienced exactly the same problem. However, in addition, we've also got a lot of water under the carpets, and we thought that there had been a leak somewhere - perhaps via a window seal?
We had to call the RAC, as we couldn't start the engine with the driver's door closed, but when it was open, the car started.
We had the same problem where if we had the engine running and took the keys out of the ignition, the engine continued to run!

The moment you opened the door, the engine would stop!
The RAC man took a look at all the earth connections, and electrical components that he could see, and couldn't see anything wrong with them. Then, he lifted the carpets, and looked for cabling under the seats, and noticed that there was lots of water under the carpets! The car base was soaking. He thought that water had entered the car somehow - perhaps via window seals or doors? Then, it was assumed that water had been the cause of these electrical problems.
So. With the keys removed from the ignition, and the driver door closed, the dash lights come on at half brightness, as if there's a dodgy connection. This takes power, so unless I want a flat battery, I have to disconnect it.
So. My first question is this. Did you have water problems like mine? If not, then mine are obviously two separate issues.
I can't see how the alternator would be at fault, as the problem happens when the engine isn't running.
It's booked in to see a Chevrolet dealer tomorrow, as it's specialist electrical diagnostics that I need, opposed to a normal grease monkey scratching his head. I can't go on with this problem.
Thanks.