This only happens when I am on the expressway, especially more when I am driving up a hill or overpass. The rpms spike abruptly and the engine jerks (on another note, i need to change my motor mounts according to mechanic). For example, lets say the rpms are at 2000, it will jump to 2100 or 2400. i have changed spark plugs but nothing. I have always used super unleaded but I had changed to mid grade for a few months. When I changed back to super, it stopped doing it for a day. But soon afterwards, it started doing it again. Like I said, it only happens when ac is on. I have turned it off and driven over the overpasses and nothing happens. While driving in the city, I never have this problem. Only when I drive over 50 the least. Thanks for any assistance.