2000 Mercury Marquis car will not run

My car will start and idle fine. Sometimes when I put it in gear it seems to bog down and die. It keeps this up until I gun the engine in neutral then it runs fine for the rest of the day. It may run fine for a week or more then it goes back to the earlier problem. I had the ford dealer run a computer check and they said everything was ok. I also had them to put new plugs in the car. Still have the problem periodically. Help