Hi Bob,
Haven't determined yet if it is the origin, a part of, or neither any of both are part of the problem, but why did you set the quality of your clouds to 438 samples which is equivalent to a quality of 6.88??
That's very very high. I see you've set your atmosphere samples to 16, which is very low.
The reason you've probably set cloudsamples this high is because of noise, but looking at these settings the noise is caused by too less atmosphere samples rather than cloud samples.
I'd go for atmosphere samples ~48 and cloud samples ~80. Same quality result and 15% faster rendering. Just checked that with 640x480 renders of your file without any models.
Those renders finished without any problems by the way, but haven't tried the full res yet, because lack of time.
Martin