Originally Posted by
Peckish
You said something that gives me a clue - it doesn't happen when you make straight cuts. I don't think it's the blade, I think it's the fabric. You are cutting it on the diagonal, which means bias and stretch. Are you using a ruler? I would make sure you cut only one layer at a time, hold the fabric down with a ruler, and cut slowly.
Also check the board condition... If you normally use the marks on your board to measure and cut you can develop straight ridges and then when going across the borad you could effectively be cutting over a few mountain ranges.
If that isn't the issue, try weighting down the other side of the fabric with a second ruler leaving yourself a channel to cut aling the edge of your main ruler with a second one a few 8ths away to hold the fabric down.