acceleration question

[bart]

Hi, I'm new in this forum and my car (ford contour 98 2.0 4 cyl, auto, 108 000 km) is new for me. I bought this car a month ago and I have 2000 km on it. The car is driving well bit sometimes happen this thing: when I want to accelerate fast on highway car first stall, after that changing gears and going ahead, but sometimes changing gears and so slowly accelerate(looks like there is no power on it ). So, do you have any idea what is going on and my I check something?