Kevin, have you found the answer? My experience has been mostly due to one of two things. Either one side of the material is being pulled by the operator or the piping itself. I have switched to the hollow, clear piping that I get from Keyston bro. Have had little issue with this on our marine projects. If your using that and still getting the wavy welt, it will be because there is a pull on one of the two pieces your sewing together.

Are the feet walking high enough?

I’ve learned depending on material (fabric/vinyl) pay attention to sewing on the Bias of that material. Sometimes you have to feed the welt in and other times just letting the machine do the work while paying attention to stretch and pull.

Also like above stated make sure you have the right size feet for welt. Then also adjust pressure feet pressure and thread tension. If you notice you’re sewing with a 6mm stitch length and it’s performing a smaller stitch length then it’s gathering and causing the issue.