This issue is caused by a faulty heating element in the fuser. Not the film.
I just point this out in case anyone comes across this, save you the time of trying to change the film.
HI, HP 4250/4350 you just clean the fuser film ,check that the left eddge of the film is clean and make ground with a brush type wire ,clean the eddge of the film with file paper ,thats all. GOOD LUCK