Sounds like dodgy fuel (it can go off when stored for long periods). The volatile components of the fuel (the most combustable compounds) can evaporate off leaving less potent compounds, therefore lower octane. Try some fuel conditioner. May up your Octane rating enough to stop the surge.

If that doesn't work, it may be one for Kev's vast expertise.

BTW: Is Markie Dans still in business? Did my best to keep them busy back in the day :-)