engine problems

I have a Renault 5, Campus 5, 1990, mileage 145, 000. It has been acting up. Every time I need to stop or slow down the engine dies. I have no problem turning it on again, but it seems that as soon as I move my foot off the accelerator, the engine dies. the mechanic checked the carburettor, and found nothing wrong. what could be the problem?

thank you