Maybe the screw that holds the bar in place isn't making the right contact to hold it in place. The bar should stay stable enough to keep it from moving about. I am not sure if you can take a walking foot in to have it examined. While the fabric is out of the machine does the foot wiggle about? Try looking at it with a magnifying glass to see if something is not aligning up. There are a few tiny bars and screws in most walking feet. Maybe one is crooked? Bent? I don't know your foot enough to know what could be making it not work right.
Another thing you could try is seeing if the shaft bar that holds the foot is not functioning right. Try regular fabric and a straight sewing foot to see how the straight line looks sewing regularly.
I hope you can determine the problem. It is frustrating to have a foot suddenly act up.