Lack of power on motorway

I have a 1998 1.25 fiesta,83k miles,starts at the touch of the key,ticks over perfect and runs around town fine.When i take it on the motorway it takes miles to get from 60 to 75 and then slows at hills.It will run fine when it is up to speed ( even at 80+).I have changed air filter and fuel filter and have tried the handbrake test on the clutch,have owned the car from new and would like to keep it running for a little longer,any ideas most welcome,many thanks