why do my vauxhall astra 1.7 diesel cut out ?

hi could any1 help me i have a vauxhall astra 1.7 diesel it runs for a couple of miles but then it cuts out and sometimes it starts back up after a while. when i drive it, it seems to lose power and sticks on 3000 revs and when im in 1st gear i put my foot all the way down and dont seem to give it anuff revs to pull off in first gear ive changed the throttle body but still doing the same anyone could help ?? thanks