Electrical problem
1998 Buick Lesabre 6 cyl Front Wheel Drive Automatic 52000 miles

My mother-in-laws Buick runs fine but occasionally won't start. Usually this occurs after it has been driven for a short distance, but once occurred on initial attempt to start for the day. Sometimes waiting a few minutes works, another time we jumped it and it started. Battery has been replaced, positive battery connection has been tightened. The problem is it always starts when we have it at the mechanics. Therefore, since it is starting, they can't tell what's wrong and there's nothing to fix.

Thanks!