I used to get this with my front slipping out, but only up-wind. The cure was two things.
First was like Ola says, as I was slightly under down hauling my sails. Now I always down to max. But you do this already, so no action needed there.
Second was to tighten the front foot strap so my foot felt locked to the board. Only down side is jibing, now I need to slip my foot slightly out of the strap just before jibing, over wise it used to get stuck (made me look silly)
Also, I know you say that you have tried different foot strap positions, but, front strap at the back & inboard wont work with back strap middle and out board. First it will make down wind stance feel weird, second it will make upwind hard as you cant get out board as much. Also, at 178cm, it sounds like you've got a very wide leg stance, this will make you put your power through the back leg, leave the front kinda floating!!
Try Front strap outboard/back and rear strap outboard/forward (or front strap inboard/back and rear strap inboard/forward). This will close your legs and make it easier to distribute your weight through both your legs.
Just my 2 cents worth, and hope you figure it out.