Sounds like it could possibly be one of two things. A possible faulty ignition switch, or something is going on with the security system.
If the you have a red light for the security system, does it blink while the vehicle will not start. It may be an immobilizer that is kicked in, or possibly the ignition switch is intermittantly inoperative.
〉 Answered on Nov 24th, 2007 by Amanda J. Pierce, MBA "AJ", Certified Mechanic, Project Coordinator; Primavera Scheduler at VIGOR Alaska Industrial Shipyard & BizzM3ch Solutions
Does the vehicle actually "crank" when you have the key turned? Does it make noise like it
〉 Answered on Nov 21st, 2007 by Michelle Wingard, Automotive Expert at AskPatty.com, Inc.
It sounds to me like it may be a key or tumbler problem. If you have a second or spare key, try using that for a while and see if it does the same thing. If it doesn
〉 Answered on Nov 20th, 2007 by Suzanne Grego, Technician at City of Philadelphia Fleet Management