Duncan, I think you're right on when thinking you ran out of volts (PWM maxed out). I wrote a post on my blog about that not too long ago with calcs for a Warp 11 motor and 100v system if you're interested. Just look about half way down. There's a guy from the Ukraine who keeps asking me about that setup in a Porsche 924 like mine. That's why I decided to run 230 volts in my car.
Porsche 924 Electric Conversion: The Need for Speed