mgf over reving and power problems

hi all i have a 2001 mgf 1.8vvc i just fitted a new engine to it however there is a problem with it the engine revs up to 3000 rpm and then when it settles down i go to take it a test run and its jerking back and forward like lack of power or fuel once i press the throttle ive fitted new plugs and still doing it does anyone have any ideas to the problem thanks in advance

from the last news http://ift.tt/1mRicHG

via IFTTT