I'm going through the same issues as we speak with my B6 S4, and my garage thought it was the immobilzer also, then when checking codes, nothing...so they're leaning towards the starter also....if you've already rebuilt yours and seen no real results, I'll pass your information on and see. Does your S4 not start more often in the mornings?or after you leave it for 6+ hours without driving it? It seems like mine takes almost 15mins and some praying and then, like there as nothing wrong it just turns over???It's like chasing a ghost...then after it starts and I shut it off and come back to it starts right up..???? It's has been getting worse for the last month...so I'll need to get it fixed before it plays dead forever. If you get any insight on your problem, can you pass it along? I'll do the same once my garage gets it within the next couple weeks.