Cyclic? That leans me towards the BCM. If it were random I'd lean towards a door switch. Try disconnecting the negative battery cable for 5 minutes to reboot the BCM and it's CCD data bus. To save
some battery power remove the door lock relay on the fuse panel under the dash. Course then you have to lock manually.
It will cost you a couple hundred but a shop will connect a scan scope and ID the problem. Here's an example with relative to you system info though it's not quite your problem:
http://www.youtube.com/watch?v=UFZqZB1Eds8
This is a 3 part video. I gave you part 2. You can go to YouTube and look at parts 1&3 if you like.