whats wrong with my car ?

can any one help i have a ford st170 2003 when i change gear and put my foot on the gas pedal it over revs when clutch is fully up this also happens when i am driving along when i put my foot down it revs up about 750 revs more than i should then comes down the car is also lacking in power it has just had an oil change on friday runs alittle better but still doing the above