I have a similar problem. At first I had automatic clock updates turned on; the clock set itself 2 minutes ahead after a certain while, then 4 minutes ahead,.. I have another phone set to update teh clock automatically, the same provider, but never any issues with time.

I disabled automatic update of the clock and set it manually. It was okay for a while, but today it was again 2 minutes off. Must be a problem within the device.