Buick Forum - Buick Enthusiasts Forums - 05 Rainier- Battery going dead

I am new to this site and have a question concerning my 05 Rainier. It has been taking spells of running the battery dead in about 7 hours. Have replaced the alternator and the battery and it keeps on doing this when it wants to. It has been to a very reputable garage and they keep telling me they cannot find nothing wrong with it. When it takes a spell of going dead,we jump it to get it started and it will start and run fine for maybe a week to three weeks. Sometimes it will run the battery so dead it will not even light a dome light. Once we jump it or charge the battery, it will start and run fine till it decides to do it again. This problem has been very frustrating because everything supposedly test out fine. If anyone has any input as to what we might be missing, I sure would appreciate a response. This problem has been going on for about 6 months and getting about ready to trade it off because we cannot trust it to start when WE want it to instead it starting when it wants to.We have absolutely loved this vehicle till now. It has 80,000 miles on it.