My 98 Disco I (109K miles) has started losing significant power at freeway speeds. It happens primarily at RPM < 3000 under acceleration and the engine will just start to run rough with no power. If I push hard on the gas usually it will downshift and the engine fully powers up, or if I back off and back on the pedal power returns. My mechanic checked the fuel pressure and it was good, and I just installed new plug wires and recent new plugs. Check engine had misfire noted in two cylinders and my mechanic reset it, and it did not come back on until 50 miles later even though the frequent power cutout problem kept recurring. I drove it to Tahoe this weekend (400 miles) and the problem just kept coming up, but did not seem to worsen much. Any ideas here?

 

Speed sensor will do this every time

 

My 96 disco does the same thing, and I have concluded that it may be a sticky exhaust valve- typical Seafoam cleanout may not work, and I may be facing removing the heads and doing a valve job. I will keep following this thread to see others' input.