AVES
I had a similar problem last year. Like your friend I looked at cables etc to no avail. About a week before the problem manifested itself I had over-filled the engine oil. In talking the problem through with local club members, I investigated how the roller barrels operated and noted that on opening the throttle (engine off) a black, sticky residue could be see on the roller.
It was suggested that this could be as a result of oil blowing back through the head. Whatever the cause, I cleaned it all up and car has been fine ever since
hope this is of use
Rog