Hmmm, that is very odd. When you select straight stitch and move the needle over what does your read out show for stitch width? If it is anything other than zero, you need to manually change it to zero. Perhaps the act of moving the needle over is making the machine think you want to zig zag. Changing your stitch width to zero will tell it you want to straight stitch.
I have never had this happen but I have 153QE that is about 15 years old.