My 1986 SS stock 305 has a surging problem after you have driven for about an hour. I live in Phoenix where it is hot most of the time. When I am driving a constant speed it starts to pulsate, up-down, up-down a few rpms over and over. If you speed up not present, if you slow down, not present, at an idle not present, only when driven at a constant speed. Any help would be greatly appreciated.