That is one nice design (and build)! At first glance, it has a short tail moment arm and a powerful engine/prop on a quite long front moment arm, so the prop might destabilize the model in the yaw axis. If that is the case a dorsal fin or a bigger vertical tail could help. That's speculation, though, I didn't put that model in a stability calculator.
What I would expect is a problem with the wing airfoil. It looks like true flat-bottom (e.g. Anderson SPICA), what is of course customary for these small models. These are great airfoils for slow flying models but not for a fast model. At lift coefficients of 0.45 and lower, there is a bottom side stall producing much drag. And as always, the airflow departures happen to be on one wing first (randomly), making for a strong yaw moment.
A fast model like this one flies at low lift coefficients, say 0.2 or so. You wrote it flew very slowly on the maiden so that could have been normal (top side) departures (stalls) randomly left or right. If you get the engine running properly, you could try normal slow flight and see if the hunting is still there and then fast flight and see if it is there again. Just a thought...
Last edited by UStik; 11-28-2017 at 12:49 AM.