When I'm accelerating through the gears the car feels hesitant and judders/struggles a bit, like there's a flat spot or some loss of power. It happens in 2nd and 3rd at around 2000rpm, speeds between 20-30mph. The speedo and tacho needles tend to dip when it occurs as well. It's fine in 1st gear and clears above 30mph and in higher gears. I'm not 100% but my impression is that it only starts happening after the car's warmed up, and not when it's cold just after starting. Could it be something with the fuel mix that gets hidden by the auto choke? i have changed plugs, oil and air filter but that didnt fix the problem. help please!!?

thanks in advance