I had the same problem the other day and nothing showing on the instrument panel to show an electrical or engine management fault. Took the
and plugs out one at a time and on the last one the plug was soaking with water. Dried the plug and the cylinder with a hair drier and it's been fine ever since. Not using any water either and nothing points to the head gasket, so touch wood so far so good. Have read on a couple of other sites that the seal between the scuttle panel and the bonnet is poor and water can work it's way down from there if it's been especially heavy rain or through a car wash! Someone also suggested that leaving the engine running while getting dragged through an imo car wash might not help. Draught sealer tape along the top edge of the bonnet seems to be the recommended fix. Might not be your problem but it's worth a check!
Hope you get it resolved mate!