keeps cutting out?

hi to any1 who can help me, i have a spear&jackson petrol grass trimer 33cc, when i pull the cord to start the trimmer it works fine after a couple of pulls it will start, but after about 20 to 30 seconds it cuts out, it will also rev up and the trimmer head will start spinning but then it will just cut out, i then pull the cord and it will start straight way then cut out again, the helpline in the manuel dont work either so i got no manufactor support and argos when i got it wont help cos i lost the receipt.... thanks for any help