car starts slowing down?

car starts slowing down put your foot down on the throttle to go faster and nothing happens only when i lift my foot off the throttle ad put it back on it goes no prob it is a vauxhall zafria 1.8 16v changed the ecu throttle body m.a.f. lambda sensor. the is also no engine light showing when it has this prob also my father works in a vauxhall dealer and he cant find what the problem is