when you are sitting still at a red light car has a slight flutter

Although occasionally the final test in a diagnostic routine really is "replace with known good part" it sounds like you need to find a different shop that is actually willing to do diagnostic work. Be forewarned, however, real diagnostic work is not easy and it isn't cheap. This is probably why a lot of shops resort to just swapping parts. In part it's because a tech may be too lazy or inexperienced to know how to test for certain faults, but it's also really hard to get a customer to understand why they should pay $600 when all that turned out to be wrong was something like a broken wire. They don't see the eight hours spent trying to figure out where the broken wire was.