Jane-E,
Thanks for the response. Please see the response I sent to Derek above. With respect to your questions:
My leading is set to: 15pt.;
Align to Grid is set to: 'all lines'; and
Increment is set to 15p
The problem manifests due to the multiplicity of fonts employed for the elements interspersed within the 'main body text'. In order to have these elements (some hand-written fonts, some typewriter fonts, some email fonts) appear somewhat authentic, their internal leading needed to be something other than 15pt. Combine that with the fact they are of unique and different sizes anyway, that throws the main body text 'out of sequence', so to speak.