This is completely normal behaiour for self launching gliders, The high mounted engine produces a strong nose down couple. Ali's ASK 21 has a nosewheel which is a good choice for this kind of application. The Fox has no nosewheel but you could always add one.
Careful use of the throttle is necessary on take off. Try starting with the lowest possible throttle setting that will get the model rolling and hold in full up. as the model accelerates the elevator will begin to bite and then start to slowly add more throttle. The big danger is then having the model rotate with full up still applied so try to reduce it as you go. However, I have never had a powered Fox , just a pure glider version. I say this because every different type behaves a bit differently, so there is a bit of a learning curve.
If you are not familiar with the Fox, I would councel you to use the best possible servos, and top quality linkages on the back end. Make sure that the fin cannot twist. They are reknowned for elevator and rudder flutter. Sorry if I am teaching Grandma to suck eggs...
If you get it sorted you will have a truly super model. Love mine and it's just a glider I chuck off a big hill when it's blowing 35 mph +.