Make sure the tub and sink traps are not empty when this is happening.
Sometimes wierd things happen with the vents, debris , strange winds ,
etc because not all plumbing jobs are optimal.
In addition in a very old house I have seen vent piping rusted through
and leaking sewer gases to the interior.
I doubt it is the toilet, but it could be the soil stack that rises up
through the roof .
Because it is intermittant my guess is some event is sucking the water
from tub or sink trap.

Ike wrote:
I only ask this question because I hate the thought of doing it and am
not sure it is necessary. We have a toilet on the second floor. It is
in a true water closet -- 3' x 5'. There is no sign of any leakage
around the toilet or on the first floor ceiling. However, once in
awhile, mostly when it is very hot upstairs, this tiny bathroom has an
unpleasant odor to it. This seems to happen even when to bowl has been
thoroughly scrubbed and sanitized. I am considering pulling the bowl
to see if the wax ring has some seepage. But, before I do, is it
possible that this odor is coming from a vent someplace or from the
sewer line?