So, I got the tune up done to the Taurus today.
And, although the problem is slightly less noticeable, it's still there. The hesitation is still readily felt, but it's not as bad as it was before. I had the plugs, wires, fuel filter, and air filter changed.
I'm at a loss for what it might be, now. I guess I'm going to talk to the mechanic at work again and see if he's got any other ideas...maybe a PVC valve replacement or an EGR valve replacement.
I'm starting to wonder if it's the coil pack in the car. It literally only does it at certain RPMs and certain speeds. Basically, when you are accelerating from a highway speed to a higher speed to pass a car at 4,000 RPM.
The thing that is annoying about it is, I think I've only had to accelerate to a speed at that RPM once since I've had this car, and that was the time I noticed it. So it's a problem that doesn't really have any effect on the driveability of the car...but since I know it's there, I feel like I should take steps to rectify the problem.
Maybe before I put more parts in I'll just run a couple of tanks of premium gas and some good injector cleaner into it to see if that does anything to help with the issue.
Hopefully I can get it all worked out for not a ton of money...