It sounds like coordinating the speed of your machine and the speed in which you move the fabric. Try to keep the speed the same, then when you move the fabric around, change how fast you are moving it around until your stitches come out even. Then try and remember and practice moving your hands that same speed.
Some have mentioned placing something in the pedal so that they can only press it down the same distance each time. That eliminates one issue, so you can focus on the speed your hands are moving the fabric