Hey.....there are A LOT of BT issues.....
search under BT Keybord in this forum and you will see that they have a time out problem as well.
BT headsets are not easy to get to work, the N900 is really picky about what types that are accepted it seems.
I have used a NOKIA BH-503, worked fine for music, but voice& mic quality was "muffled" like I had a pillow in front of me..
do more searching here and you will find info regarding all the BT issues that N900 do have.