David,
Me, too. Are you squaring off the end first? And open each strip after you cut it and square it off again if it's crooked. You might need to do it as often as every 3 or 4 strips. I have found I need to put some pressure on the ruler and you will probably need to use sandpaper dots, InVisiGrip or some type of thing to keep the ruler from skidding around. It's also supposed to help if you walk your hand up the ruler as you cut (stop cutting when you walk your hand). My quilt instructor suggested cutting fabric doubled once and held up selvedge to selvedge and then moved back and forth so the fold lies even. Then is when you lay it on your mat and then square it off. Use your longest ruler and a small square next to it to even it up, then move the small ruler out of the way. I hope you can visualize what I'm saying. Good luck with it, it has gotten easier for me.