Possibly the fuel filter. Maybe your system can't sustain the pressure at higher output.
It's really hard to say. I take it that giving it more gas doesn't help?
It could be your torque converter lockup. Perhaps without the "slip" of the torque converter, you just don't have enough oomph to sustain the speed without more gas. The converter clutch locking up normally reduces the RPM's suddenly -- but it doesn't usually result in a loss of speed.
Or it could be that you are running into an invisible force field? Perhaps a cloaked Warbird is parked next to the highway?