renault megane coupe sport?

hi mate, up untill yesterday my car was fine.after about 20 mins of drivein the car seems to lose nearly all its power and plods along very slowy, and sounds diffrent, then it will just go back to normal, but not for long though. ive changed spark plugs and leads but still just the same.could it be them two things that sit on engine that the leads plug into, opposite end of spark plugs. please help.... thankyou... graham