Hi,My name is Tim,
I will do my best to assist you
Mark,the first thing to do,is to buy an alternator with a lifetime warrenty,here in the US, auto zone and most parts stores offer a lifetime warrenty ,,this way you wont need to pay for another if it does go bad,,,
after you have the new alternator installed,do a load test,if you dont have a load tester,again,most parts stores will do a load test for free..this is to make sure the lternator is good under load,,,you must understand,the replacement alternators you are buying are rebuilt and not new,,sometimes these will have problem.
if you arent blowing any fuses ,i wouldnt think there is anything wrong with the car itself,..just bad rebuilt alternators