You may have a trouble code being set and every time you disconnect the battery, the code is cleared until the vehicle is driven and the code is set again. Is your check engine light on before you disconnect the battery? You could also have a problem in a module or have an electrical problem. I do suggest you have this vehicle diagnosed soon by a qualified technician that can run some tests to find the problem.
〉 Answered on Apr 17th, 2008 by Suzanne Grego, Technician at City of Philadelphia Fleet Management