If your AC Compressor is seized, do you still have a belt running to it? If so you might want to eliminate the belt going to it. When I bought my 240 two years ago this had already been done. My Volvo mechanic advised me that this is not a problem despite the fact that two belts normally drive the alternator. Definitely stay away from the WD-40; it is a penetrant and will shorten the life of your belt. Their are spray on belt dressings which help the belt(s) grip the pulleys and work pretty well. When running one alternator belt I highly recommend changing it every two years (Trust me on this one). Your problem should be solved with cleaning up your pulleys with a solvent, replacing your belt, and a little belt dressing if needed. The alternator belt can be very tempermental. It will definitely need some seasonal tightening.