Fuel pressure regulator? Idle CO set properly? Four teeth present on the crank pulley?

edit

scrap idle CO - it's green top and not adjustable. therefore - lambda sensor been replaced too?


oooh ooh - air leak? split vaccum hose?

I have a twincam Sierra that does exactly the same thing and I've never got to the bottom of it, despite replacing everything but the engine! WOuld love to find the answer to this one too