Firebird Scouts fly just fine...have flown 3-4 of them, getting kids into the hobby.

A few ideas:

1) The instructions do tell you, iirc, where to balance the plane. (The CG, or Center of Gravity) If the plane is nose diving, it's very likely it's simply nose heavy, and moving the battery a bit, or adding some weight to the back, will resolve the issue. In fact, the wire you've added to the boom could even be having an impact...remember, this is a VERY light plane, so even a tiny amount of weight, especially if it's well forward of the CG, can make a big difference. Try using some CA to hold the boom in place, and remove the wire and/or any tape you may have added.

2) This aircraft controls altitude and pitch proportionally with throttle input...i.e. no elevator. If the battery is not fully charged, or is not holding a charge, insufficient thrust will be produced, which could cause the aircraft to sink. I've also seen one user put the prop on backwards, so it certainly can be done.

3) At full throttle, the Scout should just about fly out of your hand...it doesn't need much of a toss at all. Tossing it TOO hard could cause unexpected behaviour on launch. I would THINK the behaviour wouldn't be consistent as you've described, but ya never know.

4) Almost without exception, these planes come out of the box WAY out of trim. Have you adjusted the v-tail control surfaces to make sure that they are in line with the v-tail itself when the control stick is in the middle position? Double check this, as even a small amount of misalignment can have an impact on a plane as light as this one.

Don't give up on this bird yet. While they're not what I'd call 'great flyers' by any means, they're certainly a flyable and enjoyable introduction to the hobby.