It sounds as if your cat may be meeting up with some other contaminated animal. Front line is probably the best product out there, it interupts the flea life cycle.
I think the best bet would be to get a flea comb and dip into soapy water (soapy water is like a mild insecticide), remove all the fleas, give all the places the cats go a thorough vacuum and treat with a mammal friendly spray or powder. if flease reappear on the cat then you can be assured some other animal is the source.
P.S watch those silly sprays/powders you get for the cat, they are harsh and can cause the cat to fit , i have witnessed this horrifing experience on more than one occasion.