Al:
Thanks for your input. On Saturday we spent a while testing matters - and discovered a strange source of the problem. The bulb has a spot where the reflective material is absent. It so happened that that "hole" in the reflective coating caused light and thus heat to be aimed right at the grounding spring. By pulling out the light receptacle slightly, we are able to move the position of that hole enough so that the light no longer trips the GFCI. Clearly a temporary fix - I'll replace the bulb when I open the pool next spring. BTW, this is 120V, 500w lamp.