sounds like a vaccum leak to me. when it pops off and loses the vacuum up at the throttle body, the car will stall and die. i had the same thing happen to me. CEL came on and the code said the engine was running too rich, aka getting too much air.
dont take it back to the mechanic. get under the hood and make sure all of the rubber hoses and hose clamps are on the endes of the pipes/MAF/throttle body and tighten em down. this will give you a little wrench experience and save you some cash to put towards
!!