Thanks for the responses. The GFCI receptacle is in a bathroom. When
it is working, I've run a heater with no problems. The wires are run
through the attic. I spoke with my electric company yesterday and was
advised it might be a loose ground connection and that maybe it only
happened after a heavy rain. I don't remember the previous conditions
but Thursday we had heavy rains and the GFCI tripped and wouldn't
reset, but Friday afternoon, after things had dried out, it reset fine.
Now I have to find where the ground connection is.