Ruudje:

The c/g on mine is at 2-5/8", the plans call for 70mm 2-3/4".

IF you had yours at 75mm 3" I believe your aircraft was tail heavy by your desciption of its flight behavor.[&o]

Hope you can salvage the airplane and if so go back and rebalance at 2-5/8 - 2-3/4" w/o fuel.

Best of luck with future flights.