I recently purchased a 89 deville and after 300 miles on the expressway at 70mph (with cruise on or off) I started to notice a shutter or vibration when the car would start to go up an incline. It would only do it right at the start of the incline and would not act up again untill the start of another incline. At 55mph I did not notice the problem. Could this be a fuel problem? Does anything think I might need to replace the fuel filter....I am sure its the original and there is 128k miles on the car. Any other info would be appreciated.