BEA55T E

I've got a 2006 E55 and the instrument cluster stopped working today. For the past few days car has been stating key not recognized and I would have trouble turning the key. Today I replaced the batteries in the key itself and took my daughter to school. On the way home I stopped at the store for a few items. Got in to start it up and key would not turn so I pressed the lock and unlock buttons and tried again and it worked. As soon as I put it in gear all the dash lights went out and the a/c stopped. The window rolled down but would not go back up. Got home parked it. I tried to start it but everything was dead. Once again I locked and unlocked the car and got in and it started right up with no problem and gauges worked fine. wen t back out 1 hour later and it the same thing all over again. What could this be?