Sounds like your shutter or aperture are not functioning properly (overexposing the picture). It's no doubt repairable but it would probably be cheaper to upgrade. Can you post some examples with EXIF data? You might also try setting to the lowest ISO setting, and using M with as fast a shutter speed as you can set. Then try using several aperture settings and see if you get any difference. Then set as small an aperture as you can, and try varying the shutter speeds. If your constant shutter speed arrangement gives you best results on one of the shots, your aperture is the problem (probably stuck open); if varying the shutter speed gives you the best picture, the shutter's having problems. Once you find out which does best, you'll be able to work around the situation. I'm guessing it's the aperture sticking just from your description of what, and when, it goes off.