i guess i can think of something else after all,
are you sewing blocks together with the paper still attached? i tend to ignore straight grain when paper piecing and leaving the paper on will help to stabilize the block until it can be attached to its neighbors for stability.
hope some of this makes sense to you. it can be so frustrating to have a problem that makes us crazy.
maybe someone else can come up with something. if we put our heads together i'll bet we can help you :)
good luck, let us know how you are doing.