18volts from alternator

i have an 83 ranger with a gm alternator on it i forget the amps i know that it is the common gm alternator though just in the last 3 months or so ive gone through 4 alternators that all are doing the same thing. after having them on my truck for about a week they start slowly putting more voltage out then eventually it will start to put out around 18 volts but not always it will go between normal and 18 volts. i had this gm alternator on my truck for about 2 years with no problems till now. all the alternators came from the same store. any ideas on what to check or what could be causing this problem would be great

i would be suspicious of the voltage regulator. You may want to get that checked out.