String Trimmers

My Husqvarna 128LD has recently started rotating while in use. The engine and trimmer head are no longer in alignment. The rotation has slowly been advancing during use, and has now reached the point that the fuel filler cap is pointing straight down during trimmer operation, resulting in leakage and erratic operation. There's no obvious loose point to rotate the unit back into alignment (and then tighten it all back down). How do I rectify this?