calling all mechanics

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i have a problem with my car which is driving me mad and was wondering if anyone has any suggestions. when i reached about 50mph the car would slow and be struggling for power everytime i needed a bit of extra power such as when over taking or going up hills. i had the air flow sensor replaced and now in higher gears its great. the problem now is when it is in lower gears such as pulling off from junctions or round abouts. again it struggles for power. i have had it serviced, a new lambda sensor and new fuel filter. i am fed up with taking it to the garages and being charged a fortune for them not to be able to mend it. has anyone got any suggestions its a lovely car when it doesnt have this problem so dont want to get rid of it please help!!