Why won't car start after it has been running for awhile, after parking briefly?

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After driving for a brief period of time, if you stop the car (e.g. to run an errand) when you get back in- it will not start. The engine revs but never turns over. It has always started back up but sometimes after a long 20 minutes. We have been back to the dealer twice- but still are having the same damn problem $3000.00 later. They replaced the fuel pump & filter, crankshaft sensor, intake boots, disa valve, vent valve and idle valve. Please help!!