Intermitant Violent Steering Wheel Shaking at speed

[Layfs]

Hi All

Have a really weird intermittent fault that happens about twice a month.

The issue is very similar to the poster in the following link (annoyingly they never informed the culprit).

http://www.volkszone.co.uk/VZi/showthread.php?p=6574329

The issue seems to happen when on long journeys using the cruise control.  I suspect its something electrical (possibly power steering pump) but am really stumped.

The wheels are balanced, tyres are pretty much brand new, had tracking checked and all is fine.

The real puzzler is it doesn't happen at a specific speed.  I can drive for 10-20 miles at 55mph but on longer journeys (50+ miles) this can happen.

Any advice? 

Thanks

[MoMartin]

Have you ruled out a warped disc? After a while the disc will heat up and warp even more, which will cause a shake

[Bellend]

Sticking caliper at a guess.