Need advice on my 1998 1.8 petrol?!?

I was driving my Freelander for an 40 mins or so at about 100km/h when it started to just lose power in 5th gear. I dropped it 4th and it was ok for a bit but then same thing happened, the more i put my foot down the more power it was losing. It was ok in 3rd gear for a while but then the same thing started happening. A friend of mine said it may be the fuel filter but i have no idea....any ideas??

thanks your help would be greatly appreciated!!!

Yeah, you might want to check the fuel filter or the air filter...changed my air filter once and suddenly got a lot more power again.