I think you might be getting fooled by a bad ground. What are you using for ground when you are testing ? If you could put a light or buzzer on the boost terminal before you start working your way to the battery it might be interesting to see if/when it comes on.

__________________
1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)