This happened recently and again today. On start-up (not from cold) the engine raced erratically all by itself and 'blipping' the accelerator made no difference - it seemed worth a try under the circumstances. So, I stuck it in gear and drove anyway. Up through the gears to the end of the road by which time the engine would then idle nicely again. The car then behaved perfectly as normal. As I say, this also happened and disappeared in much the same way a couple of weeks ago.
Can't find anything obviously wrong (the ecu isn't swimming in a plenum full of water for example) so where do I start?
Oh, and yes it is one of those normally trouble free K series