Sluggish acceleration at highway speeds

[SCMark]

1997 XLT, 4.0 SOHC (20K miles after overhaul)
My symptoms are:
When driving at highway speeds push on the gas pedal to bump up the speed. Speed stays constant with no acceleration until the pedal is literally on the floor.
Once the gas pedal hits the floor the transmission kicks down and finally acceleration occurs.

Any ideas?

 

[Flash]

The throttle cable can stretch so that even with the pedal flat you don't get full throttle opening. Check out this video.

Other things to check are the air filter and fuel pressure and maybe the fuel filter.