2001 Chevy Caprice car stops in eratically

Hi,
caprice 2001 . Problem is it stops while 1)waiting in signal 2)going slow speed with out foot on accerator pad 3)sometime even at speed of 60. It never happened in high speed. Maximum time in waiting in signal or going slow in turn after lifting leg from accelarator pad. Did computer for engine. No alarm. They said difficult to find out. On doubt changed fuel pump. No use. Electronic module near spark plug. No use. Changed spark plg. No use.
any thing to next?In speed it stops and starts quickly in fraction of second.I can feel and see red alarm lights for a while. In Signal or slow speed it stops and I have to use ignition switch to restart. It starts suddenly. What iam asking is in speed at 60 if auto car stops how engines starts by itself? If electric loose connection I think engine should not start again. But it stops, give jerk and starts in a fraction of second without delay at same speed. Or mechanicaly anything to change? Is there anything like car computer program reloading?Is it helps?If my leg is on accelarator normaly if engine stops it suddenly starts. But in signal it stops completly and have to use ignition key. Anything to do with car computer?