How can I fix a Singer Futura 900's zig zag stitch width ?

My Futura 900 suddenly sews a straight looking line when I try to stitch a medium width zig zag stitch (stitch width set to 3) and sews a very erratic zig zag stitch, which gets wider and then narrower and then wider and narrower and so on, when I set the width to 6 (the widest setting). It sews a straight stitch perfectly fine. What happened and how do I fix this?