The helium flash occurs at what stage in stellar evolution?

As a G-type star fuses its hydrogen to helium, this helium will gather in the core. As a result, the core will contract under its own weight as hydrogen is being spent. The contraction causes an increased hydrogen fusion rate, increasing the temperature. When insufficient hydrogen remains in the core, the layers above are no longer supported by the outward pressure of radiation, and collapse on top of the core, causing it to contract further, and also initiating hydrogen fusion outside the core. At this point, the star leaves the main sequence, and becomes a red giant. At this stage, the core of the giant may reach critical density for helium fusion to initiate. Since the core is composed of mostly degenerate matter at this stage, there is no regulation of the fusion rate. Also, degenerate matter is less opaque to the energies produced than non-degenerate matter, so conducts them better.

Ehr, to summarize: the helium flash occurs during the red giant stage of G-type stars.