Have you tried pulling the door cards and watching the actuators when you hit the lock/unlock button with the fob? Does it only malfunction when you use the keyless system or do the locks stick sometimes using the bottoms on the doors? The keyless system is just a set of relays wired into the factory system connected to a wireless controller. I recently had to redo my keyless entry setup as the relays had burned up.
If the actuators are trying to function when you hit the button then it would seem to me that there is a mechanical issue. Possible worn out bushing or something. I know that the door lock actuators in these cars are notorious for malfunctioning in though. If they appear to be trying to work, but still not unlock/locking the doors you could pull one out, keeping it wired up, and hit the lock button. The little rod on the end of the actuator should jump. You can also push/pull on that little rod to make sure it is moving freely.
I was kinda rambling there, but its a start!
