Mercedes 280 dies when letting go of key

I have a 280 Mercedes that starts fine in neutral, but as soon as I release the key, it
dies.  I can get it going, if I put it in first and keep the key turned, but dies as soon as I
let go of the key to change gears.  The fuel pump was just replaced, as was the
thermostat.  It was running fine after it got back from the mechanic, now it's doing the
same ol same ol. It was starting fine and then would randomly die.  The mechanic kept
it for a while and no problems. It also rained really hard last night.  No idea if that is in
the least bit important,