Cranking issue

My 97 Buick Park Ave has been running fine until I
get to a stop light or stop sign and cuts off and I
have to wait 10-15 before it starts again. It used to
do it once or twice every two to the weeks but now
it's more often. Have taken to mechanic and they
can't find nothing wrong it won't cut off on them
and cranks to start with them this has been going
on for months now. I need help!!!