[wiswind]

That hose, as I recall connects directly to the vaccum from the intake.
When the motor is running, there is vaccum on that line.
The fuel pressure regulator works with MAXIMUM fuel pressure to the fuel rail with NO vaccum (like it is now, with the vaccum line disconnected), more vaccum, less fuel pressure.

Vaccum is at the maximum at idle.......and at the minimum under the most engine load......so the fuel pressure will be maximum under the hardest engine load.

You may have one of 2 problems......
Too low of fuel pressure.......so application of vaccum to the fuel pressure regulator further reduces the fuel pressure.
OR.....the fuel pressure regulator is defective.......not common, but I guess it could happen.

The next 2 suggestions are not very likely....as it should cause more trouble at idle.....more vaccum.....more problem.....lower RPM....more noticable......

Is there any sign that the fuel pressure regulator is leaking fuel into the vaccum line......there should be NO fuel pass through the vaccum line.

Another thing.....the vaccum line should fit snug on the fuel pressure regulator......not loose, letting air leak into the vaccum line at the connection.

At this point.....it is really looking like the problem is low fuel pressure.....with the fuel pump being the most likely (but also the most expensive).
With the vaccum line to the fuel pressure regulator disconnected and plugged......do you still have problems going faster than you mentioned...and issues going up hills?.......I am hoping for the fuel pressure regulator vs fuel pump, because of cost.........although it is not a common failure.....I can happen.

The thing about the fuel pressure on the windstar, there is NO measurement of fuel pressure or fuel flow....
What this means is that the PCM does not know what the fuel pressure or flow is......so you will not get any indication that directly tells you that there is any problem with either......you will only get symptoms of the problem.

There is a fuel pressure test point on the front fuel rail.....in the middle that one can check the fuel pressure and see if it is at the correct value.
You know you have some fuel pressure......as indicated by the fact that the motor starts and runs, but you can still be low.

If you have a local mechanic that you trust, you could call and see if you can stop in and have them check the fuel pressure and see if it is within specifications.
If it is not.....and since you have a new fuel filter.....it leads us to the fuel pump needing replaced.