Mine ended up being a bad output on my aftermarket alarm. I tore it out and I now much prefer my factory alarm. I guess I was one of the lucky ones that was just a fluke. Sure, the range on the alarm isn't as good as the aftermarket, but I kept breaking the 2-way pagers and even with the standard aftermarket alarm remote I kept hitting the unlock button even on the other side of the property of my work and unlocking it. I guess 2 years out of an aftermarket alarm is good, but factory is so well integrated since it uses the display.
By the way, to the original poster, if it's still acting up, go into the menu that pops up when you first start the car. If you don't already have it set, change the Lock in Drive setting to yes. If it locks and unlocks every time you put it in gear or take it out of gear, it's probably a BCM issue. Some people are having to leave this setting on to be able to lock the doors with the remote which should signify a bad BCM.