OK, I did about 5 hours' driving over the weekend so I was able to narraw down the behaviour to a very specific set of conditions.
1) When I'm driving around town at speeds up to 40 mph the empeg works fine.
2) When I join a motorway/A-road and go above 50 mph the empeg cuts out within 2 mins and displays the W disk icon. When I pull out the empeg and reinsert it, it works for about 2 mins and then cuts out.
3) When I decelerate to below 50 mph on the motorway/A-road, the empeg starts playing again.
Is this becuase the vibrations in the car are more rapid at high speeds? Driving over bumps, potholes etc at speeds below 50mph doesn't cause a problem. So, I guess it's either the cable or the IDE header (it's not the temperature, that's within the tolerance limits). Which is it most likely to be?