I've come across a couple of topics discussing the issue way back and most of them seem to point to an issue with the battery either dying or not charging sufficiently. Since you've already replaced the battery (with the correct specification for the car, I assume), perhaps have the alternator checked. Are you driving very short distances /infrequently which results in the battery not being sufficiently charged? Another option is that something is draining the battery (faulty tracking device perhaps). From what I've read on forums (not sure if it's correct), these cars have a system which limits power consumption from non-essential items, the clock obviously being one of them, to conserve battery power if the battery running low.
Perhaps John @ Leo Haese or Gert @ OBDWorx has other suggestions.
