2001 Chevy Impala Engine stops without warning

My 2001 Chevy Impala LS w/ aprox 122K miles recently began to stop when I'm driving. No sputter or other warning - almost like switching the key off. It starts right back up after coming to a stop and putting it into Park. It has done it several times in the last week, however, not at the mechanic's where it spent Tuesday. I thought maybe bad gas and tried a gas additive, but that wasn't the solution. Hot, cold, idling, running - nothing in common to the problem. No check engine lights, etc. It doesn't matter if it is hot or cold, running or idling.

If this is a repost, I apologize - also had some minor computer problems (when it rains - it pours).