I replaced the batt. on a 91 lin. cont. after 2 days i went out to start it and it was dead, whats the cause?

the new battery is testing good and the alt seems good. i jump it off then drive it and the battery seems to charge up, but sitting long periods seems to drain the batt. again. could this be a faulty voltage regulator even though the alt. tests good?? please somebody help...