1998 Saturn SL2 alternator problem

77000 miles. I purchased this vehicle from a family member about three years ago at 60,000 miles and other than the serpentine belt being replaced, nothing had ever malfunctioned. About a two years ago I started having alternator problems. The light comes on for about twenty seconds and then goes off - usually on my way to work every morning - and then two or three days later it completely dies. So far I've replaced three of them, and this afternoon I've noticed that the light is going on again - after having the new one only for two months. I had a Pioneer stereo installed in the car about the time this began happening, and took the vehcile in to Circuit City today to have them look at it again. They couldn't find any problems, and showed me that everything was plugged in without any shorts, etc. This is beginning to be expensive, and I can't figure out why hthis continues to happen. Has anyone else ever had this problem?