security -- staring problem

On Buick 2005 Century, 85K miles, red security light comes on at times
and occasionally it will not turnover or start.  When this happens lights and
wipers work.  No known battery or charging system problems.
Sometimes it starts after wiping key or waiting half hour or so.  Twice I
have had to call road service, then it does start when tow truck comes.  
There is no reason or pattern to when it will or will not start.
Our mechanic will replace the ignition switch for several hundred but
admits that it might not solve the problem.  The local Buick dealership says
they can "probably" fix it for about a thousand.  Discussion boards say you
can completely remove the security system but then other problems may
crop up.
This car is used to transport elderly handicapped people.  With winter
coming I need a high level of reliability.  Any ideas?
Thanks -- Herb