1990 Other Fiat Models juddering/kangaroo

Other Category problem
1990 Other Fiat Models Engine Size unknown Wheel Drive Type unknown Manual 26081 miles

I have a fiat 126 bis when the radiator fan cuts in the engine over-revs and the car kangaroos. It drives fine on short trips but as the engine get hotter on longer trips it happens quite a lot. Please advise this is my first car, and dont want it to poorly. Thanks rachel