the slot in the back plate may have rotted out, if so get a bolt and 2 washers fit a washer over the end of the bolt pass through the spring shoe and back plate put other washer over bolt use a nylonnut and do up just enough to apply tension on spring, will work till you get a new back plate
i had a miss fire on my xk8 which turned out to be a very small water leak dripping onto a coil pack, of cause when the engine was hot the evidence was evaporating