2001 Subaru Outback Wagon

This vehicle has been running great. Just recently, while asending a mild grade on a freeway, I began to lose power. Now, the car seems to run normally until it reaches the 40 mph range, then it will not accelerate as it should. IT will gain speed very, very slowly, however, if you hit an incline in the road, it will lose power to a point well below 40 mph before the accelerator pedal seems to respond again. THere are no noises from the transmission. The engine runs smoothly without any misfires, etc. Any ideas where to begin with this problem?

Has the fuel filter ever been replaced?

The TPS Throttle Position Sensor may not be working right. It should provide electrical resistance that changes smoothly as the throttle opens and closes.

I just changed it today and it seems to be working fine now. We shall see what occurs. THank you.

I will check this out. Thank you for the lead.