Well, a lot of us (not all, I'm sure) have missed one key component to his transformation, and it fixes the torso stability problem. (It's right there in plain sight too, which makes it all the more of a *facepalm* kinda moment.)
See this circled part?
Well, it goes here (the piece marked tab in the circled area, the smaller arrow means it's supposed to face straight up.)
The black tabs sticking out of the back (you'll see them when you look on the back of your figure) rest upon the hood piece and and it literally locks it all in place. Just to make sure, I flicked him and he held together just fine.
Once you try it on your own, you'll see that it was intended. It's just not shown in the instructions, because let's face it, it was easier to just throw crap together and print it.
"We're missing a few steps here."
"Huh? Ah. Screw it. They'll figure it out. Besides, if they break it, they'll buy another one."
NOTE: It may feel like you're about to break the piece when you push it forward (because it has a catch and locks in place) but it will not break. It will make a nauseating clicking sound though. lol
And thanks to Ryan for posting his pictorial of Bumblebee, without it I wouldn't have been able to get the right pictures. Also, I will be cross posting this info to other boards, and I will link the edited photos from my Photobucket account, but full credit will be given to Seibertron.
