Have a chat with your local car spares shop as it sounds like carbon build up on the pistons causing 'overrun' ie: Engine still fires due to the carbon being so hot it causes re-ignition. I had a motorcyle that did that and I used to drive it for about 1/2 mile [just for fun] with plug leads off eventuallly it stopped when the carbon cooled through air cooling and no spark to keep it going.
Eventually your engine would stop without stalling as the carbon cools off. Motor shops sell what I think is red ex it's an engine cleaner. Be warned it don't half give off a lot of white smoke, I know I've done it. Then again follow what the shop advises as things may have changed since 1997. also again they may have something as an additive to fuel which would prevent any re-occurance.
Bob
Bob
February 2006