dave, phil from Melboune currently had a similar problem (last couple days, this forum). It is possible your coils contain electronic components such as capacitors/transistors, as well as the expected copper windings. The electronic components (especially transistors) are heat sensitive. If run beyond their heat rating they fail, but will allow a weaker spark which is insufficient . When finally hot the transistor will not function once damaged. So the unit may run until that temp is reached and stop. In the early days of electronic ignitions this was fairly common in cars. If this applies in your case the best test is to substitute a good coil. Hope that doesn't mean buying a new one as they are expensive if this discussion does not apply.
