Thanks for all the replies.  Everything seems to be working OK now.  Since I
did everything that people suggested (100 nanofarad caps across Vdd and Vss
close to the PIC pins, carefully checked for any intermittant shorts, and
put huskier caps on the 7805 and checked the output with the scope) it is
hard to say just exactly what did the trick.  Assuming the trick is did, of
course.  From what everyone has said there is no way that anything that I
did can hurt, though, in any case.

I have to say that I am amazed to learn that the caps on the regulator
(input to ground and output to ground) can be that large!  I have always
assumed that the specs of the manufacturer had to be followed pretty
closely!  Live and learn.  That is, IMHO, a Good Thing To Know!  I shall
file it with my many other Good Things To Know.

I also noticed that Bob Axtell said that the symptoms were "absolutely
classic" for the problem being oscillation, and Spero Pefhany has the
opinion that it sounds like something else (latchup, or intermittent short
of an output to supply rail.  So two of the experts disagree.  :-)  Of
course, I don't know what it was.  What I do know is that it seems to have
been cured, and I am grateful for the comments.  I tend to stay awake
thinking about these things when they do not get solved before bedtime.  :-(

Bill
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