1.5 Auto...rev's jumping and seems to be using petrol???

Hi, seem to be havin some troubles with my 1.5 auto…every few minutes when driving, the revs go mad, jumping from say 2000 to 1000 then back up 3000, quite wildly…the car also judders at the same time, taking my foot off the pedal seems to stop it for a while, 3 mins or so later it will do it again. Also seems to be using petrol quicker than normal Anyone have any ideas what the problem could be???

thanks in advance