Damn', your story is exactly the same as mine. Pro-ject told my retailer it has to be the motor of the turntable too. I understand why it could be the problem, in physical and electrical terms (the turntable motor would ask too much intensity to the speed box, which burns in consequence), but what I don't understand is that, in both cases (yours and mine) the speed box failed when the turntable is off. So, when no voltage is delivered to the turntable and no intensity is sought by the motor. You see what I mean ? I'm not sure if I'm very clear...
So I have my doubts about Pro-ject explanations.
And a turntable shop has taken a look on my turntable and everything seems to be fine, they said.
Another explanation I've been told on a french forum is that when people in a block or a neighbourhood turn their lights off and go to bed, the electrical voltage cominq in your own house is increasing. Less global demand makes a higher individual voltage (tell me if this explanation is not clear enough, it's not easy to a french guy to explain that kind of things !). Could be a solution, but it is impossible to verify and/or prove.
And, if the motor is really the origin of the problem, I really really really wonder how comes that Pro-ject built defective motors that burns pieces of equipments they sell. Building stable electrical equipments is something pretty easy in the world of electronic devices, so I don't understand.
I asked myself if it has something to do with the fact it is not a simple Debut III but a Debut III USB. Bu I don't see why this should be a problem, because in both cases (Debut III and DEbut III USB) it is driven by 16V voltage.
As far as I'm concerned, my story hasn't come to an end yet because the retailer is very slow taking care of the problem. I asked for a simple refund because it's too long (in total, it has been since last september I bought this speed box which is always dysfunctionning). I'll let you know. If you can keep me i touch too, it would be cool !
Best regards. I'll give you some news soon.