Need some suggestions on troubleshooting an intermittent door lock problem on my 2008 C300. About once every 10-15 tries the doors cannot be locked from either the SmartKey or the central lock switches on the inside of the doors. Same problem with doors unlock, but much less frequent.

What I have observed:
1. The problem began after the car sat idle in my garage for several weeks.
2. The car started up easily and continues to start easily.
3. The battery voltage measured at the test post near the battery compartment is 12.2 volts when the problem is present.
4. However, if, while the problem is present, I connect my battery charger, raising the battery voltage to about 13.6 volts, the lock and unlock functions will operate from either the SmartKey or the central lock/unlock switches on the doors.
5. Or, when the problem occurs, simply insert and immediately remove the SmartKey and the the locks will operate from both the SmartKey and the door switches.

I have not taken the car to the dealership.
Any suggestions?