I was given this Silverado on trade for some work. When I got it nothing worked. so far it's running some what ( plenty of room for improvement ). it will start and run but will not stay running with out giving it throttle. if put into gear doesnt won't to stay running either ( gets much worse ). the engine is in good heath I have changed the plugs and wires and the camshaft position sensor. I have ran high octane fuel and injector cleaner through it and it still isn't working the way it should. What should I try next to fix the problem?