Punto 1.7D (1995) throttle problem

My Punto S 1.7 Diesel (1995) has a problem: Around midrange RPM the engines'power drops down, and just above that range the power is rapidly increasing. Driving in a traffic jam is very
difficult, since i don't want to bump into the car in front me. I drive like a rocket which isn't very pleasant. Does someone has a solution for this problem?
Thanks and regards,
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