how to fix diesel high revs?

CAR- Ford Sierra 1992
1800 Turbo Diesel,,
i have just fitted new timing belts and a 2nd hand fuel pump,car starts and runs ok,,
but when you leave it ticking over for a while the revs go up to about 2500 to 3000 for about 10 seconds then returns to normal tick over.
any ideal...........
thanks