Does jiggling the key make the engine turn off? At any rate, if the tumbler is going, replace it soon. If the tumbler jams and you can't turn the ignition, it makes replacing it a lot more difficult - they'll have to drill the tumbler out. That, plus you'll be stranded wherever you are.

Fuel pump relay could be a cause, bad/worn solder joints will fail open intermittently, cutting off fuel to the engine. Happened on my 300E a few years ago, and a new one was the fix.