2006 Honda Civic Intermittent electrical failure

After being parked for several hours the battery appeared to be dead (no response to key, no power locks, no lights, no starting, etc.) Without intervention, several hours later, everything worked fine, car ran normally. Drove for 30 minutes, parked for 10 minutes, started right back up again. Immediately drove for another 1 hour and then *immediately* after turning off ignition, no power to anything, no lights, no locks, would not restart. 1 hour later, starts fine again. Any idea what could be causing seemingly intermittent electrical failures? They seem to occur both whether the car is hot or cold, and the battery terminal connections appear clean and intact. Otherwise no problems with car. Battery is relatively new (~1 year old).