Strange Bug

[kaffiene]

A have a lit, coloured mesh which flickers when drawn. When I turn of lighting and just draw it without shading, there's no problem (just draws solid white)

I thought this might be a driver problem but it appears the same running on different machines as well.

The flickering affects only the lit mesh (background textured but unlit skybox is unaffected).  The mesh sometimes shows just ambient lighting, sometimes is lit properly and sometimes appears appears full white.

Does anyone have any idea what might be going wrong?