The flex tube on my isuzu (mind I have a 3.2) is under the middle of the car after the cats.
Can this cause a stumble? I don't think so but it can cause it to run a little richer and you will smell it and the engine will be louder.
As for a computer most shops will have these.
It will allow to see things like the cycles of the O2 sensors to see if they are healthy and it can look at voltage at ind coils.