Hmmm, do your strips look wavy before you sew them together? Or is it just afterwards that they look wavy? I was sewing a load of strips together a few weekends ago, and after I finished sewing them I thought they looked terrible and wavy, but when I set the seam and then pressed to one side they looked fine.
Could you maybe show us a photo so we can see it for ourselves, as that may help with coming up with ideas for you? I wonder if its the step of ironing the strips after they've been cut that is causing the problem? I don't think I've ever ironed a strip after cutting, I tend to do it before cutting and then after sewing, but then I do iron as little as possible if I can help it! Also, are you pressing or ironing, as that could make a difference I reckon!
I am terrible at cutting strips and just got myself the sizzix 2 1/2" strip die and am in strip cutting heaven! I finally have consistently cut strips!
I do hope you find a solution soon, as it is no fun to be so frustrated by a hobby!