Sounds consistent with starter and/or solenoid wearing out. But many posts have noted problems with the ignition switch contacts - which are easily replaced - and give a variety of erratic symptoms when bad. The common test seems to be wiggling the key in the ignition. Also if you did not yet, may be worth looking at the connections right at the starter itself, and for the possibility (though remote) that the starter is loose. In over 400,000 miles so far on 3 volvos I have yet to have trouble with a starter. Doesn't prove it can't happen though.