fiesta 1.1 running probs

have an L reg 1.1 fiesta very clean cars starts first time but after renning for about 500 yds sometimes it will not rev up loses all
acceleration ticks over etc but will not rev up when driving at 30 mph in top gear i know they have no guts but will not accalerate it nearly cut out on me this afternoon but when i stopped and restarted it was okay for a while??? any one got any ideas i have a feeling its the carb but not sure have cleaned cransensor connections but no difference please help
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