Thanks Edward.

Battery is OK
Leads to the SCRs are good.
Yes, does it with drive wheels in the air.
The brushes look adequate.

I haven't checked the cable leakage yet.

Does the speed controller go thru any kind of staged increase?

I ask because once full speed is hit, (instantly after the same brief normal period), the controller dumps the main contactor.

This seems consistent with something causing full speed to occur followed instantly by the controller maybe recognizing an out of control situation, like a stuck ON SCR, and dumping the contactor. A puff of smoke always leaves the contactor after an event. What I'm saying is this whole event cycle is extremely repeatable. Accelerator depressed only until motion starts - normal PWM whine starts. This continues for about 3 seconds followed by full speed - bang! - no speed.