vauxhall vectra 1.8 1998

[aceone]

when i start the car from cold the vacuum pump comes on an is excesivly noisy ...when i pull away the car is very sluggish then the pump seems to cut in and out for about 100 yds...then the noise of the pump stops and everything seem prety much ok,,allthough sill a little hesitant....can anyone possibly suggest what the problem may be....thankyou[size=18][/size][size=12][/size]