My 1982 300SD develops a stiff "gas" pedal after it is warm. Sometimes after I have been driving for a while, the pedal becomes almost impossible to push, unless I mash it to the floor....this does not feel like the right thing to do. After I mash it, naturally it down shifts and accelerates. If I am in traffic when this happens, I am going fast-slow fast-slow...and pushing smoke for no reason due to the pedal mashing.....vacuum???...where should I start?