a couple of things... forst, that airplane would need to be EXTREMELY nose heavy for it to lawn dart like that... what is much more likely is:
1) the elevator moves in the wrong direction (when you pull the stick BACK, the TRAILING EDGE of the elevator should move UP)
2) the elevator is trimmed down too much (with your hands off the sticks and the radio turned on, the elevator should be trailing edge neutral / streamlined OR SLIGHTLY trailing edge UP)
3) with the bixler, you should launch with only 1/2 to 3/4 throttle the first couple of flights, with the down thrust and high mounted motor, full throttle tend to push the nose down.

to check the CG:
the CG should be set at 60mm from the leading edge of the wing, measured where the wing meets the fuselage....
make a mark on the bottom of both wings, 60MM back from the leading edge.
with the battery in place, put your fingers on those 2 marks and see if the airplane's nose drops (nose heavy) , tail drops (tail heavy) or stays basically balanced on your fingers (balanced)
if it's nose or tail heavy, try to move the battery around forward or aft until the airplane does balance.

my personal thought is that you had a combination of the above issues...