I have a 2007 Cadillac SRX. For some reason recently the key cylinder periodically won't turn all the back to the off position. I read that this was because of a weak battery and sure enough a day or so later my battery went dead and the alternator gave up the ghost. I replaced the alternator and battery this weekend, but today it did the same thing again.
Anyone else had or heard of this problem before?

I haven't heard of it related to the battery and/or alternator.

It sounds like the lock cylinder is getting worn out.