ozandy...Sounds strange I know.. but....are you placing you battery on the bird exactly the same everytime....they are very sensative to CG...

Unless something in the linkage is wobbly and worn out I would say something has to be moving on you...like your fly bar?......but first make sure your battery is loaded in the same place front to back and side to side....and then trim it well....take two asprin and call me after the third fllight...