98` 4x4 Ford Explorer Sport 4.0L SOHC Starting problems

My Explorer from time to time has trouble starting.When I trun the key it revs up and just stops. Like a Varoommm then stops.The motor dosent catch and runs and cuts off its like having a vacum cleaner in the on position and putting the plug in the wall and pulling it out quickly.After I wait a few seconds it starts and runs fine.The dealer couldn`t find anything wrong but dirty fuel injectors .I have my fuel injectors cleaned every forth oil change. Other than this problem the truck has never had a problem for 8 years.Any ideas?