Why does my paragraph break before and after a subscript?

RH 9.0.2.271, Word 2010, Windows 7, 64-bit PC

This is an intermittent problem occurring in the Word output. The WebHelp is fine. In the following image you can see that the subscripts worked in the early part of the paragraph, but then split.

Here is the html:

<p>where <span class="EQN_Char">q</span>_{<span class="EquationText">tot</span>}

is the total rate of heat transfer (which you supply), <span class="EQN_Char">k</span><sub><span

  class="EQN_Char">c</span></sub> is the laminar thermal conductivity

of the primary fluid in the cell, <span class="EQN_Char">ρ_c</span>

is the density of the primary fluid in the cell, <span class="EQN_Char">C_Pc</span>

is the specific heat capacity of the primary fluid in the cell, <span

  class="EQN_Emphasis">V_c</span> is the volume of the cell,

<img src="HX_EQ2a.jpg" alt="" style="border: none;" border="0" /> is the

magnitude of velocity in the cell, <span class="EQN_Emphasis">T</span><sub><span

  class="EquationText">ref</span></sub> is the reference temperature

for the heat exchanger, and <span class="EQN_Emphasis">T</span><sub><span

  class="EquationText">c</span></sub> is the temperature in the cell.

The summation in the denominator of equation 1 is carried out over all

the cells in the heat exchanger. Thus, it can be seen that the sum of

the weights for all the cells always add up to 1, so that the distribution

scheme ensures that the sum of heat transferred to all the cells in