My suggestion stems from identical symptoms to Erik's occurring with a Bosch starter on a well-used '64 Alfa 1600. Once the knock on the solenoid did the trick several times, I took off the solenoid cap in the way George describes. The moving copper bar inside had rattled sharp-edged grooves in the sides of the plastic cap, and sometimes got stuck in those grooves. Hence no click. I smoothed out the grooves, rounded the edges of the copper bar, and dressed the contact surfaces like George describes. No further troubles. Volvo engines may not vibrate their solenoid innards the same way, so it's a long shot, what I suggested.