Losi xxx mid motor nosedives?

[Ferret]

Today I finally completed my Losi xxx mid motor conversion and took it for testing on the the track.
I was really surprised how well it handled "out of the box" with my guesswork setup.

However, there was a fly in the ointment; it had a strong tendency to nosedive off ramps unless I launched with a lot of throttle, to the extant of actually flipping forward many times.
The car uses an extra gear so that the motor is spinning in the same direction of the tires.

Did anyone else with a conversion experience this?
Any ideas how to resolve it?

Thanks,
Avner.