Passat "Jolt" at crusing speeds?

My 2002 Passat has a rare, but disturbing, tendency to jolt while crusing at highway speeds. I can set the cruise control and drive without problem for an hour, then BAMB, a one second jolt that feels as if I've run over a 2x4. It almost feels like the wheels lock for a moment. I don't think it is not a problem with the engine; the jolt is too fast and too violent to be any kind of engine hesitation.

Any ideas on where to begin?

Thanks,
Tom

Could be your control arms (usually upper). I feel jolt when dryving on highway when releasing accelerator. Raise your car and try moving wheel and if it feels loose.

I have a Volkswagen Passat 1.9 TDi , manuafacture year 1997. Recently it started to jerk when driving over 50 miles p/h.
What could cause this?