How far in do you place the pinch rollers? I put them about 1/4" in from the edges of the vinyl. That way if it gets skewed a little bit- it's still going to cut okay.
Another thing that might help is slowing down the cut speed. About 30 cm/sec or lower- like 20 cm/sec is better. If you are cutting very large letters (around 20" tall or so), then you can slow it down even more.