I was not able to make any more headway on my own, so I bit the bullet and took it to a shop. I ended up at Housekamp's Auto Service in Grand Rapids, Mi off Division Ave South. They have been very reasonable on pricing, especially considering it has been there over a week now.

They found a few things:
* The timing was off by two teeth (thank god for non-interference engines), also I haven't touched this, meaning it's been running like this for more than a year :O
* There was a large vacuum leak around the 1st cylinder injector as I removed the plastic caps that came on the new injectors thinking they were protectors and nothing more. Turns out they hold the o-rings in place.
* The distributor cap was 180 degrees off (I didn't know it could even be put on wrong, unless they meant the way that the firing order was laid out.)

After diagnosing and fixing these issues (mostly caused by me) they found it ran a lot better, but the sputter above ~3000 rpm didn't go away. After making sure everything mechanical checked out, they looked at the computer and found some corrosion (not sure how much) in the connector. They tried cleaning it, waiting a day before reconnecting it, and still haven't had any success in eliminating the hesitation. On Monday (9/23), another guy they know that used to work on older Volvo's is going to look at it. If they aren't able to fix it, I'll probably end up having to put a standalone ECU on my car (which I wanted to do anyway, but was hoping to wait a while). If it comes to that, I will document the process on this forum.

Thank you all for your time and ideas on what to try. It means a lot to a novice home-mechanic like myself.

- Nathan