electrical mystery

every once in awhile we discover the battery (otherwise ok) has gone dead in our '97 960 wagon. when the ignition is turned to the starting position, there is a rapid clicking sound but the engine won't turn over. a jump-start gets it going again -- until the next time it happens, sometimes weeks or even months later but always mysteriously. one past clue has been the discovery of a fan running somewhere inside the car when we climb in. the fan, location unknown since no rush of air can be felt, seemingly has turned itself on and run-down the battery. the only other clue, if it's a clue at all, is that the light/s which illuminate the position of the automatic shift lever -- reverse, drive, park, etc. -- flickers on and off unpredictably, suggesting a short somewhere. we assume that at the bottom of our mystery is, in fact, a faulty switch or a short which turns on fans or who-knows what else. we had a volvo-oriented garage check this awhile back but their high-tech equipment could detect no electrical problem anywhere (but it did accept my triple-digit check for its troubles). now the problem is back again. are there any wizards out there who have confronted any such hobgoblin? if so, thanks for any suggestions. art