Hi
I have worked on a AJ with a 700 motor and later a 800 motor .These boats will run good when set up.
I'm wondering just what kind of coupler you are using. The most successful set screw couplers have a lot of screws that bare down on the flex shaft. The ones I most often make use nine screws. A single set screw can work if you do everything right . Use Blue Loctite on the set screw. Solder the end of the flex shaft about a half inch. File any excess off. File or grind a small flat on the soldered flex shaft where the set screw will make contact. These things if done will let you get on with running your boat. Either .125 or .150 flex is just fine with 700 power and 12 or more cells.
In watching your video ,it look like you boat is just barely hooking up after a long start. You prop may not be right to get you on plane. What prop are you using?
n.h.schmidt