2004 Vauxhall Combo starting

hi every one my first post iv just got a vauxhall combo van 1.3 2004 when i got it the fella told me it had a starting problem he said it was getting air in the fuel pump in the tank when i turn the key on you can hear a sucking noise from the tank then i try and start it some times it starts and sometimes it don't i have had the heater plugs checked and they are working fine once it starts it will start all day sometimes it takes a few seconds and sometimes it takes 10/20/ seconds to start is this sucking noise the problem a few times when its taken 20 or more seconds to start its running lumpy and i have no throttle i turn it of and start again and its ok or does anyone no what it could be its driving me mad also a few days ago when i was driving up a hill the service light came on and the power froze can anyone help tar