2000 impala turns over but won't start

So I've had this car for 3 years with no real issues until this last month. When the engine is cold and it's her first start, she starts like a dream. But if I drive around and say go to Walmart, by the time I go to leave it won't start. It turns over but will not get the "vroom". I replaced the spark plugs and wiring component as well as the ignition switch. When I took it into GM it didn't happen for them at all. I don't want to go drop more money into this giant guessing game of what could be the problem. Has anyone else had this issue and if so what fixed it? Thanks in advance!

3.8 or 3.4?

your right just throwing parts at the car seldom repairs the car. Do like you are going to WalMart but go home instead. you need to find out if its is fuel or spark when it wont start. I assume that eventually the car will start. I would take off the air intake and squirt some sensor safe throttle body cleaner in the throttle body and see if the car will turn over . You also need to check for a spark when the key is turned if the car does not start. Harbor freight tools has a inexpensive spark tester.