The problem only happens after the brakes are warmed up and yes, it does get worse the more I use the brakes. They are Mopar rotors from Autowest Dodge. I replaced them a few months ago and all was fine with them until a few weeks ago. There are several times in my drive home where I encounter slowdowns, going along at 70 then down to below 20 in short distances. I suppose it is quite possible that the fronts may have warped.