I have a 1996 GMC Jimmy and when the truck shifts into 3rd gear there is some hesitation and sputtering...usually around 40mph. When I accelerate up to around 55-60mph the hesitation goes away. What could be causing this? A transmission going bad, maybe? I don't really notice any other problems, the truck shifts from gear to gear just fine and there is no real jerking or lag when I shift into drive. The truck has 240,000 miles on it. Any help would be greatly appreciated!