renault scenic engine problems?

i have a renault scenic the old type t reg 1.6 petrol 8 valve it keeps cutting ive stripped the fuel system down and blown the pipes through the return pipe on the tank centre was blocked but that hasnt seemed to make much difference it doesnt matter how you drive it the engine runs ok for 20 minutes half an hour then it just stops i turn the ignition off and start it again it goes ok for a couple of miles and it stops again its realy iritating i know nothing about petrol engines diesel im fine its the only petrol i have so any help would be apreciated