I doubt it's the ECT... that sensor is not very critical.
Is throttle response normal under 50%?
Does this only happen at a certain RPM or at any RPM?
Does it idle normal or rough?
Does the power cut out completely?
Does it cut out abruply at a particular throttle position?
Is the check engine light on?
Possibly the MAP sensor (manifold absolute pressure) on the airbox. Injectors are a possibility... but your mileage seems too low for clogged injectors. Could be a bad PCV valve. It might be a problem with the throttle position sensor. It could even be something in the ignition.
Here;s what happens in most engines above 50% throttle.... at lower RPM's, going past 20% throttle does not increase the amount of power the engine puts out since 20% throttle is plenty of air for that particular RPM. At hgiher RPM's, the engine will switch over from a more econmical fuel/ignition map, ot one that provides better performance. This is why your mileage drops so quickly when you have a lot of passengers, head winds, driving at speeds over 70mph. In those conditions, the engine is no longer trying to run lean to conserve fuel, it wants ot produce more power.
Hope this helps some. hard to diagnose FI problems sometimes.