Thanks for the info Gene.
It definitely did it less often today, doing the same long drive. The weather was a bit cooler than of recent weeks and it rained. However, it still did it a fair few times, particularly during takeoff/acceleration or braking. It didn't seem to do it much at all while cruising.
When it cuts out... 9 times out of 10 it will cut out for 2-5 seconds, then come back on for 0-1 seconds, then cut out for 1 second again... before returning to normal.
Can anyone else back up Gene's theory, or have any additional info?