1995 dodge ram wont start?

hi there i just bought a 1995 dodge ram. its when i bought the car it fired right up no problem. after i bought it i drove around turning the car off and on. the next morning i went to my truck and it would not start. it cranks all day just wont even try to sputter the engine. i know some what about trucks and i got the truck started by messing with the throttle when someone else cranked the engine. sometimes it starts sometimes not.. if i get it started the truck runs fine. i can start it shut it off all day as much as i want. just when it sits over night does it not want to start. i need some help on this one. its been at the mechanics for 3 days so far ive got the computer sensor changed he still does not know whats wrong. ps it has a kill switch i have to push in while i start the truck. if that matters.