FordF150 vacuum loss

Hi i have a '97 F150 6 cylinder that has been givivng me trouble.running rough and stalling when idling at times..we have taken it to the garage 3 times and so far they haven't been able to fix tyhe problem..tests show a bad sensor but when they replace it still does it..engine light is constantly on.. a friend sugeested it could be a vacuum loss and no leaky lines were found but we installed a vacuum gauge and when it starts running rough(usually when you give it gas) there is a definate vacuum loss on the gauge..please i am running out of money and ideas..by the way its a gasoline engine..someone have any ideas what could be causing it?..Thanks